High capacity optical node

ABSTRACT

An optical core node that includes optical switching planes interfaces, at input, with multi-channel input links and, at output, with multi-channel output links. The number of channels per link can differ significantly among the links, necessitating that the input (output) links have different connection patterns to the switching planes. Such an optical core node requires new methods of assigning an incoming wavelength channel to one of the optical switching planes. The assignment of the channels of input and output links to input and output ports is established in a manner that increases input-output connectivity and, hence, throughput. Additionally, the networks that may be formed with such optical core nodes require new routing methods. A preferred method of selecting a path through the core node favors switching planes connecting to a small number of links. In a time-division-multiplexing (TDM) mode, the method of path selection is complemented by temporal packing of TDM frames.

FIELD OF THE INVENTION

The present invention relates to data networks and, in particular, to a high capacity optical node and methods used by the high capacity optical node.

BACKGROUND

The envisaged future telecommunications network includes electronic edge nodes and an optical core that comprises a plurality of optical cross connectors. Currently, an optical cross connector has no practical means for storing optical signals. This severe limitation has implications regarding the structure and operation of an optical-core network.

Wavelength-division-multiplexed (WDM) fiber links, each carrying several wavelength channels, connect the edge nodes to the optical cross connectors. The optical cross connectors may also be interconnected by WDM fiber links. In multi-hop routing, individual wavelength channels routed through the network may traverse several optical cross connectors. Such multi-hop routing within the optical core precludes the use of time sharing scheme, such as TDM (time-division multiplexing) switching or burst switching, due to the difficulty of coordinating the switching times among the bufferless optical core nodes.

In a switching node having a plurality of input ports and output ports, where signals arriving at the input ports are generally uncoordinated in the time domain, being generated by independent sources, and have arbitrary destinations, contention resolution is required because input signals arriving at two or more input ports may require connection to a single output port. Contention resolution can be based on a primitive method where one of the signals competing for an output port is accepted while the remaining signals are blocked, i.e., denied a path through the switching node. The blocked signals would be discarded if the respective input ports have no storage facility. Otherwise, the blocked signals can be buffered until given permission to proceed at subsequent intervals of time. Alternatively, contention is avoided by scheduling the transfer of data from input to output.

An edge node that establishes a path through a core node is said to subtend to the core node. An edge node may subtend to some or all of the core nodes of a network. When an edge node subtends to two or more core nodes, it selects a core node to establish a path to a given destination edge node according to some merit criterion. The edge nodes sending data traffic through a core node are said to be subtending edge nodes of the core node.

The path availability for signals arriving at a given input port and destined to several output ports of a switching node do not necessarily occur according to the same order in which the signals arrive to the switching node. Thus, a simple sequential storage facility is not useful to deploy, and, currently, there are no practical means for buffering incoming optical signals and selectively retrieving any signal for switching to a respective output port when a path across the switching node becomes available.

The use of interconnected optical cross connectors in the core of the network leads to an inefficient rigid network. Traversing two or more optical nodes is problematic for the following reasons:

-   -   (1) As described above, the absence of buffers makes time         sharing of wavelength channels virtually impossible.     -   (2) The required paths for edge-node pairs must be established,         through the core, one at a time. Resources must then be reserved         during the path setup phase, which may take a considerable         amount of time, several-hundred milliseconds for example. During         the path setup phase, the reserved channels are kept idle.     -   (3) Paths can be blocked due to mismatch of links of sufficient         vacancies. For example, there may be several candidate routes,         each having two or more hops, from a source edge node to a sink         edge node through a plurality of interconnected optical cross         connectors. If each candidate route has one link that is fully         reserved, a free end-to-end path can not be found even when all         other links in the set of candidate routes are lightly loaded.         This is conventionally called mismatch blocking. To reduce the         mismatch probability, the occupancy of the optical core must be         kept at a relatively low level, which can be realized by known         internal-expansion techniques but may be considered costly.     -   (4) Wavelength conversion is often needed in multi-hop optical         paths.

To circumvent these difficulties, an obvious solution is to disallow the use of consecutive optical nodes in any path. With this approach, complemented with time coordination between each core node and each of its subtending edge nodes, the buffers at each edge node can be exploited for contention resolution. A time-locking technique is detailed in Applicant's copending U.S. patent application Ser. No. 10/054,509, filed on Nov. 13, 2001 and titled “Time-Coordination in a Burst-Switching Network”. With time locking, all edge nodes subtending to a core node can time their transmission to arrive at the optical core node at any desired instant of time. Time locking is feasible when a path between two electronic edge nodes traverses only a single optical core node. This enables adaptive channel switching at the optical core nodes without the need to allow large idle periods between successive path changes. Time locking also enables the use of time-sharing schemes; TDM or burst switching. It also allows the network core to operate at a high mean occupancy, 0.95 for example. Without time locking, time sharing of network transport resources is not possible, and channel switching would require that the core be operated at a lower mean occupancy, of 0.7 for example.

A complete network is defined herein to be a network that can allocate paths of a total capacity of C (bits per second) from any set of channels collectively having a capacity of C (bits per second) and emanating from any group of source nodes to any set of egress channels, collectively having a capacity that equals or exceeds C, and belonging to any group of sink nodes. For example, a source node can transfer its entire traffic to a sink node of at least equal capacity. Therefore, any spatial variation of traffic loads can be accommodated and the need for traffic engineering is virtually eliminated.

The capacity of a node is defined herein as the total data rate that can be switched from the input ports to the output ports of the node. The degree of a node refers to the number of ports in the node. In an asymmetrical node, where the number of output ports differs from the number of input ports, an input-side degree of the node refers to the number of input ports and an output-side degree refers to the number of output ports.

An efficient wide-coverage high capacity complete network that does not allow traversing two consecutive optical nodes would take the form of a composite-star network, or a plurality of composite-star networks as described in Applicant's U.S. patent application Ser. No. 09/624,079 filed on Jul. 24, 2000, and titled “Multi-dimensional Lattice Network”.

To be complete, the composite star network must be uniform, having identical optical-core nodes and identical edge nodes. To enable wide coverage, each edge node must have a high degree, having several links to the optical core, and each core node must be able to accommodate a large number of subtending edge nodes.

To enable a high capacity, each edge node must have a large number of input ports and a large number of output ports, with each input or output port supporting a channel of a high bit rate, 10 Gb/s for example. The edge node should also be provided with a high-speed controller to enable fast scheduling that can follow traffic-intensity variation. A versatile high capacity edge node is described in Applicant's copending U.S. patent application Ser. No. 10/025,982, filed Dec. 26, 2001, and titled “Universal Edge Node”.

A complete network, as defined above, responds gracefully to wild spatial traffic variation. However, this may be realized at the expense of some added optical-link mileage, as explained in a paper titled “Comparison of two optical-core networks”, authored by Blouin et al., Journal of Optical Networking, Vol. 1, No. 1, January 2002.

If the spatial traffic distribution can be determined with a reasonable level of confidence, then it would be possible to relax the strict requirement of network completeness and employ, instead, an incomplete network that is still tolerant to considerable spatial traffic variation.

It is desirable, therefore, to find a structure of an incomplete network that employs edge nodes of different capacities and degrees, and core nodes of different capacities and degrees, while still realizing a high capacity wide-coverage network that also provides high performance under diverse traffic conditions. An optimum incomplete network may include edge nodes of widely varying capacities and degrees and core nodes of different capacities and degrees to reduce fiber-link mileage. There is a need to realize such a network without significantly affecting network performance and agility.

Thus, a non-uniform incomplete composite-star network structure that can realize network economy and control simplicity is required and methods of route selection in such a structure are needed.

SUMMARY

A non-uniform, incomplete composite-star network allows for single-hop connections between edge nodes of different capacities. The creation of such a network may require the use of a non-uniform optical core node that includes switching planes of differing coverage indices. A coverage index for a given switching plane is based on the number of input links and output links with which the optical switching plane communicates. Unique methods are utilized to assign channels from input and output links to specific input and output ports of switching planes and to efficiently use the switching planes to satisfy connection requests from source edge nodes of diverse nodal capacity.

In accordance with an aspect of the present invention there is provided an optical core node including a plurality of optical switching planes, the optical core node interfacing, at an input side, with a plurality of input links, each of the plurality of input links carrying at least one wavelength channel, and interfacing, at an output side, with a plurality of output links, each of the plurality of output links carrying at least one wavelength channel, wherein at least two of the plurality of input links carry different numbers of wavelength channels and at least two of the plurality of output links carry different numbers of wavelength channels, where the wavelength channels of the input links are connected to the input side and the wavelength channels of the output links are connected to the output side so that at least one of the plurality of optical switching planes is a full-coverage optical switching plane connected to each of the plurality of input links by at least one wavelength channel and connected to each of the plurality of output links by at least one wavelength channel and at least one of the plurality of optical switching planes is a partial-coverage optical switching plane connecting to a subset of the plurality of input links and a subset of the plurality of output links.

In accordance with a further aspect of the present invention there is provided an optical core node in a wavelength division multiplexed network wherein links between nodes carry wavelength channels. The optical core node includes a plurality of demultiplexers, each of the plurality of demultiplexers receiving at least one wavelength channel on a corresponding input link, a plurality of multiplexers, each of the plurality of multiplexers transmitting at least one wavelength channel on a corresponding output link and a plurality of optical switching planes including: at least one full-coverage optical switching plane receiving at least one wavelength channel from each of the plurality of demultiplexers and transmitting at least one wavelength channel to each of the plurality of multiplexers; and at least one partial-coverage optical switching plane receiving at least one wavelength channel from fewer than all of the plurality of demultiplexers and transmitting at least one wavelength channel to fewer than all of the plurality of multiplexers.

In accordance with a still further aspect of the present invention there is provided a method of connecting a plurality of channels to a plurality of ports of a switching node, the quantity of the plurality of channels not exceeding the quantity of the plurality of ports, where the plurality of channels are arranged into M groups, M>1, each of the M groups having a specified number of channels, where the plurality of ports are arranged in N rows, N>1, each of the N rows having a specified number of ports and where each of the plurality of ports connects to at most one of the plurality of channels. The method includes sorting the M groups in a descending order according to the specified number of channels, to give a plurality of sorted groups indexed from 0 to (M−1), sorting the N rows in a descending order according to the specified number of ports, to give a plurality of sorted rows indexed from 0 to (N−1), considering each of the plurality of sorted groups in order from most channels to least channels, where considering a given group includes assigning a channel from the given group to a selected port, where the selected port is an unassigned port in the row having the lowest row index of the plurality of sorted rows. The method further includes repeating the considering until the row of index (N−1) has been reached, examining the sorted groups sequentially starting with the sorted group of index 0, scanning each of the plurality of sorted rows in a cyclic discipline starting with the sorted row of index (N−1) and progressing towards the sorted row of index 0 and assigning one unassigned channel from each group having at least one unassigned channel to an unassigned port until all channels of the groups are assigned.

In accordance with another further aspect of the present invention there is provided a data structure for determining port vacancy in an optical core node that includes a plurality of optical switching planes. In an asymmetrical core node, the content of the data structure for the input side may differ from that of the output side. The data structure includes a plurality of fields, where each field of the plurality of fields corresponds to an input port (or an output port) in one optical switching plane among the plurality of optical switching planes, where the fields are logically arranged into rows according to corresponding optical switching plane and logically arranged into columns according to an edge node from which the input port receives optical signals (or to which the output port transmits optical signals) and where the rows are logically arranged by monotonically increasing coverage of corresponding optical switching plane, where the coverage of a given optical switching plane is a measure of a number of input links on which the given optical switching plane receives optical signals and a number of output links on which the given optical switching plane transmits optical signals.

In accordance with an even further aspect of the present invention there is provided an optical core node including a plurality of optical switching planes, a plurality of demultiplexers, each of the plurality of demultiplexers adapted to receive a signal on an input link from a source edge node, demultiplex the signal into a plurality of wavelength channels and send at least one of the plurality of wavelength channels to at least one of the plurality of optical switching planes and a core node controller adapted to receive a connection request, where the connection request specifies a source edge node, a sink edge node and a requested capacity. The core node controller is operable to identify a subset of optical switching planes, in the plurality of optical switching planes, where each of the optical switching planes in the subset has an unassigned path from an input link originating from the source edge node to an output link terminating on the sink edge node, select a candidate optical switching plane, where the candidate optical switching plane has the least coverage index in the subset of optical switching planes, where a coverage index of a given optical switching plane is a measure of a number of input links and output links with which the optical switching plane communicates and indicate, to the source edge node, an identity of a wavelength channel corresponding to the candidate optical switching plane.

In accordance with an even further aspect of the present invention there is provided a method of selecting an optical switching plane to satisfy a connection request within an optical core node having a plurality of optical switching planes connecting to a plurality of input links and a plurality of output links, where the connection request includes a specified input link, a specified output link and a requested capacity. The method includes determining a coverage index for each of the plurality of optical switching planes, where a coverage index for a given optical switching plane is a metric based on a number of input links having at least one wavelength channel connected to the given optical switching plane and a number of output links having at least one wavelength channel connected to the given optical switching plane, identifying a subset of optical switching planes, in the plurality of optical switching planes, where each optical switching plane in the subset is connected to the specified input link and the specified output link, selecting a candidate optical switching plane, where the candidate optical switching plane has the least coverage index in the subset of optical switching planes and indicating to a source edge node connected to the specified input link a wavelength channel corresponding to the candidate optical switching plane.

In accordance with an even further aspect of the present invention there is provided a method of selecting a candidate optical switching plane through which to route a connection within an optical core node having a plurality of optical switching planes, a plurality of input links, and a plurality of output links, where the selecting is responsive to receiving, at the optical core node, a connection request specifying a specified source node, a specified sink node and a required number of wavelength channels. The method includes identifying a source input link, from the plurality of input links, originating from the specified source node, identifying a sink output link, from the plurality of output links, terminating on the specified sink node, identifying a subset of optical switching planes, in the plurality of optical switching planes, where the subset includes those optical switching planes that are connected to the source input link and the sink output link, selecting the candidate optical switching plane from the subset of optical switching planes, such that the candidate optical switching plane has a least coverage index among the subset of optical switching planes, a coverage index of a given optical switching plane being a metric based on a number of input links and a number of output links to which the optical switching plane connects, determining a number of available input ports, of the candidate optical switching plane, connected to the source input link, determining a number of available output ports, of the candidate optical switching plane, connected to the sink output link, determining a number of allocable paths through the candidate optical switching plane as the least of the number of available input ports, the number of available output ports and the required number of wavelength channels and, where the number of allocable paths is equal to the required number of wavelength channels, sending an acceptance indication to the specified source node indicating an acceptance of the connection request, where the acceptance indication includes an identification of wavelength channels corresponding to the candidate optical switching plane.

In accordance with an even further aspect of the present invention there is provided an optical core node receiving optical signals on a plurality of input links and transmitting optical signals on a plurality of output links. The optical core node includes a plurality of optical switching planes, wherein at least two of the plurality of optical switching planes have a different coverage index, where a coverage index of a given optical switching plane is a metric based on a number of links, including input links from the plurality of input links and output links from the plurality of output links, in communication with the given optical switching plane, a plurality of demultiplexers, each of the plurality of demultiplexers adapted to receive an optical signal from a source edge node on one of the plurality of input links, demultiplex the optical signal into a plurality of wavelength channels and transmit at least one of the plurality of wavelength channels to at least one of the plurality of optical switching planes, a plurality of multiplexers, each of the plurality of multiplexers adapted to receive a plurality of wavelength channels from at least one of the plurality of optical switching planes, multiplex the plurality of wavelength channels into an outgoing optical signal and transmit the outgoing optical signal on one of the plurality of output links to a sink edge node. The optical core node further includes a core node controller adapted to receive a connection request, where the connection request specifies a specified source edge node, a specified sink edge node and a requested number of wavelength channels, form a subset of optical switching planes, where the subset includes all optical switching planes, of the plurality of optical switching planes, that receive at least one wavelength channel originating at the specified source edge node and transmit at least one wavelength channel to terminate at the specified sink edge node, select a candidate optical switching plane in the subset of optical switching planes, where the candidate optical switching plane has a least coverage index among the optical switching planes in the subset, determine a number of available input ports, of the candidate optical switching plane, connected to the specified source edge node, determine a number of available output ports, of the candidate optical switching plane, connected to the specified sink edge node, determine an allocable number of paths as the least of the number of available input ports, the number of available output ports and the required number of wavelength channels and, where the allocable number of paths is equal to the number of required paths, transmit an acceptance indication to the specified source edge node indicating an acceptance of the connection request, where the acceptance indication includes an identity of wavelength channels corresponding to the candidate optical switching plane.

In accordance with an even further aspect of the present invention there is provided a method of generating a schedule for a connection within an optical core node having a plurality of optical switching planes, where the generating is responsive to receiving, at the optical core node, a connection request specifying a specified source edge node, a specified sink edge node and a required number of time slots in a time division multiplexed frame. The method includes identifying a subset of the plurality of optical switching planes, where the subset includes all optical switching planes, of the plurality of optical switching planes, that receive at least one wavelength channel from the specified source edge node and transmit at least one wavelength channel to the specified sink edge node, selecting a first candidate optical switching plane in the subset of optical switching planes, such that the first candidate optical switching plane has a minimum coverage index among the optical switching planes in the subset, where a coverage index of a given optical switching plane is a metric based on a number of links, including input links and output links, with which the given optical switching plane communicates. The method further includes, for a first time slot in the time division multiplexed frame and for the first candidate optical switching plane, determining a number of available input ports in communication with the specified source edge node, determining a number of available output ports in communication with the specified sink edge node, determining an allocable number of paths as the least of the number of available input ports, the number of available output ports and the required number of time slots and, where the allocable number of paths is equal to the required number of time slots, sending an acceptance indication to the specified source edge node indicating an acceptance of the connection request, where the acceptance indication includes a schedule including an identity of the first time slot and an identity for each wavelength channel corresponding to the available input ports at the first candidate optical switching plane.

In accordance with an even further aspect of the present invention there is provided an optical core node including a plurality of optical switching planes, wherein at least two of the plurality of optical switching planes have a different coverage index, where a coverage index of a given optical switching plane is a metric based on a number of links, including input links from the plurality of input links and output links from the plurality of output links, in communication with the given optical switching plane, a plurality of demultiplexers, each of the plurality of demultiplexers adapted to receive an optical signal from a source edge node on one of the plurality of input links, demultiplex the optical signal into a plurality of wavelength channels and transmit at least one of the plurality of wavelength channels to at least one of the plurality of optical switching planes, a plurality of multiplexers, each of the plurality of multiplexers adapted to receive a plurality of wavelength channels from at least one of the plurality of optical switching planes, multiplex the plurality of wavelength channels into an outgoing optical signal and transmit the outgoing optical signal on one of the plurality of output links to a sink edge node. The optical core node further includes a core node controller adapted to receive a connection request, where the connection request specifies a specified source edge node, a specified sink edge node and a requested capacity, form a subset of optical switching planes, where the subset includes all optical switching planes, of the plurality of optical switching planes, that receive at least one wavelength channel originating at the specified source edge node and transmit at least one wavelength channel to terminate at the specified sink edge node, select a candidate optical switching plane in the subset of optical switching planes, where the candidate optical switching plane has a selected coverage index among the optical switching planes in the subset, determine whether an input port, of the candidate optical switching plane, in communication with the specified source edge node is available for a particular time slot in a time division multiplexed frame and determine whether an output port, of the candidate optical switching plane, in communication with the specified sink edge node is available for the particular time slot.

Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures which illustrate example embodiments of this invention:

FIG. 1 illustrates a prior-art hierarchical network;

FIG. 2 illustrates a prior-art uniform composite-star network including an optical core populated by optical core nodes;

FIG. 3 illustrates an exemplary one of the optical core nodes FIG. 2 using a single switching plane;

FIG. 4 illustrates an exemplary one of the optical core nodes FIG. 2 using parallel switching planes;

FIG. 5 illustrates a non-uniform and incomplete composite-star network according to an embodiment of the present invention;

FIG. 6A illustrates, in chart form, the number and the nature of the intermediate switching points for connections in the hierarchical network of FIG. 1;

FIG. 6B illustrates, in chart form, the number and the nature of the intermediate switching points for connections in the non-uniform network of FIG. 5;

FIG. 7 illustrates a non-uniform and incomplete composite-star network according to another embodiment of the present invention;

FIG. 8 illustrates a non-uniform composite-star network having edge nodes of different reach-indices and core nodes of different coverage indices according to a further embodiment of the present invention;

FIG. 9 provides a simplified illustration of a network of the type of the network of FIG. 8;

FIG. 10 illustrates a non-uniform optical core node comprising switching planes of different coverage indices, according to an embodiment of the present invention;

FIG. 11A illustrates the addition of wavelength converters at the input side of an optical switching plane, according to an embodiment of the present invention;

FIG. 11B illustrates the addition of wavelength converters at the output side of an optical switching plane, according to an embodiment of the present invention;

FIG. 12 illustrates an alternative arrangement for the non-uniform optical core node of FIG. 10 according to an embodiment of the present invention;

FIG. 13 is a plot showing a desirable frequency distribution of connections between source edge nodes and sink edge nodes among the multiple switching planes of an exemplary optical core node;

FIG. 14 illustrates an exemplary input side connectivity matrix for representing input side vacancy of the non-uniform optical core node of FIG. 10 according to an embodiment of the present invention;

FIG. 15 illustrates an exemplary output side connectivity matrix for representing output side vacancy of the non-uniform optical core node of FIG. 10 according to an embodiment of the present invention;

FIG. 16 illustrates an exemplary input side vacancy data structure for representing input side vacancy of the non-uniform optical core node of FIG. 12 according to an embodiment of the present invention;

FIG. 17 illustrates an exemplary output side vacancy data structure for representing output side vacancy of the non-uniform optical core node of FIG. 12 according to an embodiment of the present invention;

FIG. 18 illustrates a highest-index-plane data structure to be used for connection setup in the non-uniform optical core node of FIG. 10 according to an embodiment of the present invention;

FIG. 19 illustrates a highest-index-plane data structure to be used for connection setup in the non-uniform optical core node of FIG. 12 according to an embodiment of the present invention;

FIG. 20 illustrates an array of input link connectivity in the optical node of FIG. 12 and an indexing table to the array according to an embodiment of the present invention;

FIG. 21 illustrates an array of output link connectivity in the optical node of FIG. 12 and an indexing table to the array according to an embodiment of the present invention;

FIG. 22 illustrates steps in a connection-setup procedure in the non-uniform optical core node of FIG. 12 according to an embodiment of the present invention;

FIG. 23 illustrates details of a channel-allocation step in the connection-setup procedure of FIG. 22;

FIG. 24 illustrates a set of vacancy data structures for use with a route selection method adapted from the method presented in FIG. 22 according to an embodiment of the present invention;

FIG. 25 illustrates steps in a connection-setup procedure in the non-uniform optical core node of FIG. 12 according to an embodiment of the present invention that uses Time Division Multiplexing and considers packing connections in time then in space;

FIG. 26 illustrates details of a time-slot-allocation step in the connection-setup procedure of FIG. 25;

FIG. 27 illustrates steps in a connection setup procedure in the non-uniform optical core node of FIG. 12 according to an embodiment of the present invention that uses Time Division Multiplexing and considers packing connections in space then in time;

FIG. 28 illustrates details of a time-slot-allocation step in the connection-setup procedure of FIG. 27;

FIG. 29 is a flow chart detailing the steps of a method for assigning channels of input links (or output links) of different sizes to input (or output) ports of a core node having a plurality of switching planes according to an embodiment of the present invention;

FIG. 30 illustrates an input-side connectivity matrix for an exemplary parallel-plane core node according to an embodiment of the present invention;

FIG. 31 illustrates an alternative input-side connectivity matrix for an exemplary parallel-plane core node according to an embodiment of the present invention;

FIG. 32 illustrates a prior-art two-dimensional, uniform, composite-star network;

FIG. 33 illustrates a two-dimensional non-uniform, composite-star network according to an embodiment of the present invention;

FIG. 34 illustrates connectivity of two edge nodes in the network of FIG. 9;

FIG. 35 illustrates alternate trails through the network of FIG. 33;

FIG. 36 illustrates possible tracks in the alternate trails of FIG. 35 indicating the multiplicity of routes available for each trail segment;

FIG. 37 illustrates a trail through a two-dimensional, non-uniform, composite-star network according to an embodiment of the present invention;

FIG. 38 illustrates the computation of track merit indices for alternate tracks of the trail of FIG. 37 according to an embodiment of the present invention;

FIG. 39 tabulates the tracks in the trail illustrated in FIG. 37; and

FIG. 40 is a track set within a single trail indicating the merit index and vacancy of selected routes according to an embodiment of the present invention.

DETAILED DESCRIPTION

A network has outer ports and inner ports. The outer ports comprise input ports and output ports. Similarly, the inner ports comprise input ports and output ports. The outer ports are connected to traffic sources and traffic sinks, and inner ports are connected to each other. The ports are typically grouped into sets, each of which comprising an edge node or a core node. An edge node includes outer ports and inner ports while a core node includes only inner ports. Within an edge node, outer input ports may communicate directly with outer output ports. Outer ports of different edge nodes communicate with each other through inner ports. The term “outer capacity” relates to the total capacity of the outer ports of a network and the term “inner capacity” relates to the total capacity of the inner ports of a network. The outer capacity is the capacity available to network users. In an ideal network, the ratio of inner capacity to outer capacity is close to one. A high ratio is generally indicative of an inefficient network.

An edge node connected to traffic sources is called a source edge node, or, for brevity, a source node. An edge node connected to traffic sinks is called a sink edge node, or, for brevity, a sink node. A link from a source edge node to a core node is called an uplink and a link from a core node to a sink edge node is called a downlink. A channel in an uplink is called an upstream channel and a channel in a downlink is called a downstream channel.

The network may have a large number, possibly several thousands, of edge nodes, with each edge node comprising a source node, that receives traffic from data sources, and a sink node, that delivers traffic to data sinks. The source node and the sink node of an edge node share memory and control. Where a distinction between the source-node part and the sink-node part of an edge node is not necessary, either may be referenced simply as an edge node.

FIG. 1 illustrates a prior-art hierarchical network 100. In the hierarchical network 100, an optical core 160, comprising several optical core nodes (not shown), is used to interconnect high capacity electronic edge nodes 140. Each high capacity (HC) electronic edge node 140 is shown to serve a group of subtending low capacity (LC) electronic edge nodes 120. High-capacity electronic edge nodes also support directly connected traffic sources and traffic sinks. Data exchanged between two low capacity electronic edge nodes 120 in a group of LC edge nodes 120 served by the same high capacity electronic edge node 140 is switched through the high capacity electronic edge node 140 serving that group. Data exchanged between two low capacity electronic edge nodes 120 belonging to different groups (i.e., connecting to different high capacity electronic edge nodes 140) is switched at a first high capacity electronic edge node 140 to an optical core node (not shown) in the optical core 160 and is switched at the optical core to a second high capacity electronic edge node 140 for sending to the destination low capacity electronic edge node 120. The number of links connecting a node to other nodes is likely to increase with increased node capacity.

The arrangement of the hierarchical network 100 allows the aggregation of traffic from low capacity electronic edge nodes into high capacity electronic edge nodes. A network such as the hierarchical network 100 is typically engineered according to spatial traffic distribution, which may be predicted or estimated from measurements. Specifically, the links between the optical core nodes (not shown) and the high capacity electronic edge nodes 140 are typically sized to meet the demands of the traffic transiting through the optical core 160. If the traffic for a group of low capacity electronic edge nodes 120 remains local to the group, a link from the corresponding high capacity electronic edge node 140 to the optical core 160 may have a relatively small capacity, with corresponding cost savings. Hierarchical structures may result in unnecessary increase of the number of hops per connections.

FIG. 2 illustrates a prior-art uniform composite-star network 200 where electronic edge nodes 220 of equal capacity are connected by an optical core 260 comprising several optical core nodes 250 of equal degree. The degree of a given optical core node 250, having an equal number of input and output ports, is the number of dual ports at the given optical core node 250, which determines the maximum number of electronic edge nodes 220 that can be directly connected to the given optical core node 250. A dual port comprises an input port and an output port. Though not all connections are shown, each of the electronic edge nodes 220 in FIG. 2 is connected to all of the optical core nodes 250. Consequently, each of the optical core nodes 250 is connected to all of the electronic edge nodes 220 in FIG. 2. The optical core nodes 250 are not connected to each other, this lack of connection between the optical core nodes 250 facilitates agility and capacity time-sharing.

Data exchanged between two electronic edge nodes 220 in FIG. 2 traverses one optical core node 250, in what may be called a single hop. In the uniform network 200 of FIG. 2, the capacities of the links connecting electronic edge nodes 220 to a given optical core node 250 are equal. The capacity of an electronic edge node 220 is defined as the sum of the capacities of its links. Since each electronic edge node 220 has the same number of links to the same optical core nodes 250, and since all links to a particular optical core node 250 are of equal capacity, it follows that all of the electronic edge nodes 220 have the same capacity. The arrangement of the uniform network 200 of FIG. 2 forms a complete network, that is, a network where an electronic edge node 220 can send all its traffic to another electronic edge node 220, of the same or higher capacity. In contrast to the hierarchical network 100, prior knowledge of traffic distribution is not required to engineer the uniform network 200 of FIG. 2.

In an incomplete network, the use of a tandem edge node may be required when data from a first edge node is destined for a second edge node that is not connected to any of the optical core nodes to which the first edge node is connected or when the first and second edge nodes connect to the same optical core nodes but the available-paths' capacity through the optical core nodes is insufficient to handle the traffic load from the first edge node to the second edge node. Instead of a single hop to the second edge node, data is sent from the first edge node to a tandem edge node that is connected to both an optical core node to which the first edge node is connected and an optical core node to which the second edge node is connected. Once having reached the tandem edge node, the data may then be sent on a single hop to the second edge node.

FIG. 3 illustrates an exemplary optical core node 250A of the optical core nodes 250 of FIG. 2. The exemplary optical core node 250A has a single switching plane 330 that connects an input side 320 of the exemplary optical core node 250A to an output side 322 of the exemplary optical core node 250A. The input side 320 connects to input links, each input link carrying optical signals on multiple wavelength channels. The numbers of wavelength channels in the input links need not be equal. The optical signal of the multi-channel input link is separated by demultiplexer (DE-MUX) 324 into its constituent wavelength channels and each wavelength channel is sent to an input port of the switching plane 330. The switching plane 330 may switch wavelength channels or bands wavelength channels. At the output side 322, multiplexers (MUX) 326 receive the wavelength channels output from output ports of the switching plane 330 and combine the received wavelength channels, or wavelength channel bands, onto output links destined for the electronic edge nodes 220. The output links need not be of equal capacity, each having an arbitrary number of wavelength channels. Naturally; the combined number of wavelength channels in the input links cannot exceed the number, P1, of input ports, and the combined number of wavelength channels in the output links cannot exceed the number, P2, of output ports of the switching plane 330. The advantage of the configuration of FIG. 3 is the ability to accommodate input links and output links of arbitrary capacities. Wavelength converters, placed at the input ports or the output ports of switch 330 enable any channel in an input link to be multiplexed with other wavelength channels in any output link. The main disadvantage of the configuration of FIG. 3 is the limited scalability, with the capacity of the entire optical node 250A determined by the capacity of the single switching plane 330. This limitation can be overcome using the conventional configuration of FIG. 4

FIG. 4 illustrates an exemplary optical core node 250B of the optical core nodes 250 of FIG. 2. The exemplary optical core node 250B has a set of parallel switching planes 430 that connect an input side 420 to an output side 422 of the exemplary optical core node 250B. The input side 420 connects to input links of equal capacity, each input link carrying optical signals on multiple wavelength channels. Specifically, each optical signal on a multi-channel input link is received by a demultiplexer (DE-MUX) 424, which divides the optical signal into its constituent wavelength channels and sends each wavelength channel to a corresponding one of the switching planes 430. The switching planes 430 are of equal degree, where the degree of a given switching plane 430 is the number of ports of the given switching plane 430, hence the maximum number of electronic edge nodes 220 that may be connected to the given switching plane 430. In the exemplary optical core node 250B, each of the switching planes 430 is an optical device for switching wavelength channels of fixed capacity. A switching plane 430 is a space switch switching wavelength channels or bands of wavelength channels. At the output side 422, multiplexers (MUX) 426 receive the wavelength channels output from the switching planes 430 and combine the received wavelength channels, or wavelength channel bands, onto optical signals sent on output links of equal capacity destined for the electronic edge nodes 220.

The text-book structure of a parallel-plane core node of FIG. 4 can be implemented in at least two forms.

In a first form, each optical space switch 430 switches input optical signals of a respective wavelength band. Recall that a modulated wavelength spreads into a wavelength band which constitutes a wavelength channel. For brevity, a wavelength band defining a channel (a wavelength channel) may often be referenced as a channel or a wavelength. The advantage of restricting each switching plane to a wavelength band is that the combining of switched wavelength channels at an output multiplexer requires no wavelength conversion, because the wavelength bands assigned to the different switching planes 430 are non-overlapping, and generally well spaced.

In a second form, optical space switching is realized using couplers. U.S. Pat. No. 5,889,600, issued Mar. 30, 1999 to McGuire discloses an alternative form of an optical cross-connector configured for use at a node in a WDM hierarchical telecommunication system that includes an optical layer overlying various electronic layers. The optical cross-connector comprises (1) a plurality of input means, to which input optical fibers are connected, each for receiving a respective group of input signals and deriving, in response to each input signal of the group, an individual optical signal of a respective given wavelength, (2) a plurality of output means, to which output optical fibers are connected, each including a plurality of receptors selectively responsive to respective ones of the wavelengths of said optical signals to provide an output signal for the output means, and (3) a plurality of coupling means each for coupling a respective one of said optical signals, each of a different wavelength, from each of the input means, to a respective receptor of each of the output means. The plurality of coupling means may comprise a plurality of star couplers, or a plurality of optical bus lines to which the input and output means may be selectively added and removed as modules. Tunable filters are deployed at output to enable selecting input signals and wavelength converters are included at output to enable sharing a WDM output link. With a suitable selection of the pass frequencies of a filter, selection of individual signals can be achieved for each output fiber so that a selective cross-connection can be achieved between the input fibers and output fibers.

Before describing the embodiments of the present invention, definitions of nodal and network connectivity parameters are provided.

The degree of an optical switching plane is determined by its number of ports. An optical switching plane having P input ports and P output ports has a degree, D, equal to P. An asymmetrical switching plane having P1 input ports and P2 output ports, where P2 is not equal to P1, can be defined as the lesser of P1 and P2. Alternatively, the degree of an asymmetrical switching plane can be defined as the arithmetic mean of P1 and P2, i.e., D=(P1+P2)/2, or the geometric mean of P1 and P2, i.e., D=√(P1×P2). Accordingly, the degree can be a non-integer number.

The maximum number of simultaneous connections from the input ports to the output ports is the lesser of P1 and P2 in unicast communications, or P2 in multi-cast communications. Thus, in a TDM switching plane, the maximum number of unicast switched data units is S times the lesser of P1 and P2, where S is the number of time slots per TDM frame.

The coverage of an optical switching plane is determined according to the number of links that can be accommodated. A coverage index of a given optical switching plane is defined herein as the number of links supported by the given optical switching plane. In an asymmetrical switching plane, the coverage index of the input side can differ from that of the output side. A “full-coverage” switching plane is realized when each input port connects to a single channel of an input link and each output port connects to a single channel of an output link. Minimum coverage results when all the channels of an input link connect to the same switching plane and all the channels of an output link connect to the same switching plane.

A switching plane having P input ports and P output ports and connecting to input links and output links each having W wavelength channels can accommodate └P/W┘ input links and an equal number of output links where └r┘ denotes the integer part of a real number r. The coverage index, G, is then G=└P/W┘. An optical switching plane having P1 input ports and P2 output ports can connect to a maximum of P1 input links and a maximum of P2 output links, each connection being limited to a single channel. With J and K denoting the number of input links and the number of output links, respectively, full coverage is realized when J≦P1 and K≦P2, and the coverage index G may then be defined as the lesser of J and K. Alternatively, the coverage index can be defined as the arithmetic mean of J and K, or the geometric mean of J and K.

In a parallel-plane optical core node supporting J multi-channel input links connecting to an input-side and K multi-channel output links connecting to an output side, where the multi-channel links may contain different numbers of channels, some of the switching planes may not connect to all upstream channels (channels on the input links) and all downstream channels (channels on the output links). Such switching planes are called partial-coverage switching planes. It is desirable, in a parallel-plane switching node, to select the input-side connectivity and output-side connectivity so that the number of switching planes providing full coverage is maximized. Full coverage allows any input link to communicate with any output link. A procedure for assigning the channels of input links to the input ports of a parallel-plane optical core node, according to an embodiment of the present invention, is described below with reference to FIGS. 29, 30, and 31. The procedure is equally valid for the assignment of channels of output links to output ports.

The definition of degree and coverage index apply to a single switching plane. For an optical core node having several switching planes, a suitable connectivity parameter is a reach index.

The reach index of a core node in a composite-star network is defined herein as the number of edge nodes that can exchange signals through the core node. The reach index of a parallel-plane optical core node equals the coverage index of the highest-coverage switching plane included within the optical core node.

The reach index of an edge node in a composite-star network is defined herein as the number of core nodes that can be reached directly by the edge node.

In overview, to accommodate varying traffic demands while attempting to minimize cost, it is desirable to allow for edge nodes of different capacities (such as those in FIG. 1) to connect to an optical core in a composite-star network configuration (such as the network configuration of FIG. 2). In a first approach proposed herein, this accommodation may be achieved by connecting some edge nodes of a first capacity to only a subset of the optical core nodes in an optical core, thereby reducing the number of links required per edge node. In a second approach, non-uniform edge node capacity is accommodated by allowing non-uniform capacity of the links between edge nodes and optical core nodes, the capacity of a link being determined by the number of wavelength channels carried. In a third approach, the first and second approaches are combined. All of these approaches are made possible through the provision of an optical core node that includes optical switching planes of differing coverage indices and the provision of an optical core including such optical core nodes, wherein the optical core nodes have differing reach indices.

FIG. 5 illustrates a non-uniform (and incomplete) network 500 that implements the first approach discussed hereinbefore, wherein edge nodes of two distinctly different capacities are interconnected by optical core nodes of equal reach index. An optical core 560 comprises two optical core nodes 550 of equal reach indices (reach index is five). Each of a set of low capacity (LC) electronic edge nodes 520 is connected to one (i.e., a subset) of the two optical core nodes 550. Each of a set of high capacity (HC) electronic edge nodes 540 is connected to both (i.e., all) optical core nodes 550. The high capacity electronic edge nodes 540 can exchange data with each other and with the low capacity electronic edge nodes 520 directly through any of the optical core nodes 550. The low capacity electronic edge nodes 520 that are connected to the same optical core node 550 can exchange data directly through that optical core node 550. The low capacity electronic edge nodes 520 that are not connected to the same optical core node 550 can exchange data by using one of the high capacity electronic edge nodes 540 as a tandem node. Preferably, traffic-distribution estimates are used to design such a network to realize network economy. Even a coarse traffic-distribution estimate can be useful.

In a well designed version of the non-uniform network 500, wherein tandem traffic is kept to a minimal level, a particular optical core node 550 interconnects those low capacity electronic edge nodes 520 that are part of a community of interest, that is, those low capacity electronic edge nodes 520 with a relatively large quantity of mutual traffic. The low capacity electronic edge nodes 520 that are not interconnected directly with an optical core node 550 may be expected to have only a relatively small amount of mutual traffic.

The switching stages for a connection in the optical-core hierarchical network 100 illustrated in FIG. 1 and in the non-uniform optical-core composite-star network 500 illustrated in FIG. 5 are depicted in the two charts 610 and 630 of FIG. 6A and FIG. 6B, respectively. The edge nodes are categorized in two types: a low-capacity (LC) type and a high-capacity (HC) type. Each chart is divided into four quadrants corresponding to connections from an LC edge node to another LC edge node, an LC edge node to a HC edge node, a HC edge node to an LC edge node, and a HC edge node to another HC edge node. A quadrant may also be divided into an outer cell and an inner cell, where an outer cell relates to two nodes subtending to the same node, and an inner cell relates to two nodes subtending to different nodes.

Only intermediate switching stages are indicated in FIG. 6A and FIG. 6B. The switching functions at the source and sink edge nodes are similar in the two networks of FIG. 1 and FIG. 5.

The symbols in FIGS. 6A and 6B are interpreted as follows: a hyphen (-) indicates a direct connection with no intermediate switching stage, the symbol ‘O’ indicates an intermediate optical-switching stage, the symbol ‘E’ indicates an electronic intermediate-switching stage.

The notation ‘O-E’ indicates an intermediate optical switching stage followed by an intermediate electronic switching stage. The notation ‘E-O’ indicates an intermediate electronic switching stage followed by an intermediate optical switching stage. The notation ‘E-O-E’ indicates an electronic, optical, then electronic intermediate switching stages. The notation ‘O-E-O’ indicates an optical, electronic, then optical intermediate switching stages. It is noted that an intermediate electronic switching stage requires two additional steps of optical-to-electrical conversion and electrical-to-optical conversion in addition to the switching stage ‘E’.

The switching stages in the hierarchical network of FIG. 1 are described below.

-   (1) A connection from an LC edge node 120 to another LC edge node     120, both subtending to the same HC edge node 140, requires one     electronic-switching stage as indicates in cell 612. -   (2) A connection from an LC edge node 120 to a HC edge node 140 to     which the LC edge node 120 subtends is a direct connection as     indicated in cell 614. -   (3) A connection from a source HC edge node 140 to a sink HC edge     node requires only optical switching within the optical core 160, as     indicated in quadrant 616. -   (4) A connection from a HC edge node 140 to a subtending LC edge     node 120 is a direct connection as indicated in cell 618. -   (5) A connection from a source LC edge node 120 to a sink LC edge     node 120 where the two edge node subtend to different HC edge nodes     requires E-O-E intermediate switching, as indicated in quadrant 622,     where an electronic-switching stage takes place at the HC edge node     140 to which the source edge node connects, an optical switching     stage takes place within the optical core 160 (and may require more     than one optical switching step), and an electronic switching stage     takes place at the HC edge node 140 that connects to the sink LC     edge node 120. -   (6) A connection from a source LC edge node 120 to a sink HC edge     node 140 where the source and sink edge nodes are not directly     connected requires an electronic switching stage at a HC edge node     to which the source LC edge node connects and an optical switching     stage within the optical core 160, as indicated by the notation     ‘E-O’ in cell 624. -   (7) A connection from a source HC edge node 140 to a sink LC edge     node 120 where the HC and LC edge nodes are not directly connected     requires optical switching within the optical core 160 followed by     an electronic switching stage at a HC edge node to which the sink LC     edge node connects, as indicated by the notation ‘O-E’ in cell 628.

The switching stages in the non-uniform composite-star network of FIG. 5 are described below.

-   -   (1) A connection from a source LC edge node 520 to a sink edge         node 520/540 (LC or HC), where both the source and sink edge         nodes connect to the same optical core node 550, requires one         optical-switching stage through the optical core node 550 as         indicates in cell 632 and quadrant 634.     -   (2) A connection from a source HC edge node 540 to a sink edge         node 520/540 (LC or HC), where both the source and sink edge         nodes connect to the same optical core node 550, requires a         single optical-switching stage through the optical core node 550         as indicates in quadrants 636, and 638.     -   (3) A connection from a source LC edge node 520 to a sink LC         edge node 520, where the source and sink edge nodes do not         connect to a common optical core node, requires O-E-O         intermediate switching, as indicated in quadrant 642, where an         optical-switching stage takes place in a first core node 550 to         which the source edge node connects, an optical switching stage         takes place in a second core node to which the sink edge node         connects, and an electronic-switching stage takes place at a HC         edge node 540 that connects to both the first and second optical         core nodes.

Thus, the non-uniform optical-core composite-star network illustrated in FIG. 5 requires less switching resources, and less optical-to-electrical and electrical to optical conversions, than the optical-core hierarchical network illustrated in FIG. 1.

FIG. 7 illustrates a non-uniform network 700 that implements the second approach, wherein non-uniform edge node capacity is accommodated by allowing non-uniform link capacity, i.e., number of wavelength channels in the links between edge nodes and optical core nodes. In the non-uniform network 700 of FIG. 7, high capacity (HC) electronic edge nodes 740 are connected to an optical core 760, comprising several optical core nodes 750 of unequal reach index, by high capacity links (solid lines), each carrying 32 wavelength channels for example. Additionally, low capacity electronic edge nodes 720 are connected to the optical core nodes 750 of the optical core 760 by low capacity links (dashed lines), each carrying eight wavelength channels for example. The optical core nodes 750 are adapted to be able to accommodate, the links of non-uniform capacity, as will be described with reference to FIGS. 10 and 12.

As described earlier, the capacity of an optical core node is the total data rate that can be switched from the input ports to the output ports of the optical core node. The capacity of a link is the sum of the capacities of its constituent channels. For example, a link comprising 32 channels, with each channel modulated at 10 Gb/s, has a link capacity of 320 Gb/s.

Each edge node has input ports, hereinafter called source ports (outer input ports), that receive data traffic from traffic sources, output ports, hereinafter called sink ports (outer output ports), that deliver data traffic to traffic sinks, input ports, hereinafter called receiving ports (inner input ports), that receive signals from optical core nodes, and output ports, hereinafter called sending ports (inner output ports), that send signals to optical core nodes. A number of sending ports can multiplex their signals onto a WDM uplink that comprises signals of several wavelength channels. A WDM downlink may have several wavelength channels which are demultiplexed and, and after optical-to-electrical conversion, connected to receiving ports of an edge node. A symmetrical edge node has a number of receiving ports that equals the number of sending ports, and each of its uplinks has a counterpart downlink of equal capacity, with all input ports and output ports operating at the same bit rate. An uplink and its downlink counterpart in a symmetrical edge node is called a dual link.

FIG. 8 illustrates a non-uniform, composite-star network 800 wherein electronic edge nodes of diverse capacities are connected by an optical core 860 comprising several optical core nodes of different reach indices, and generally having diverse capacities. In particular, the optical core 860 includes high reach index (HRI) optical core nodes 850V, 850X (referred to herein collectively or individually as 850) and low reach index (LRI) optical core nodes 855U, 855W, 855Y, 855Z (referred to herein collectively or individually as 855). The electronic edge nodes of diverse capacities include high capacity (HC) electronic edge nodes 840E, 840F (referenced herein collectively or individually as 840) and low capacity (LC) electronic edge nodes 820A, 820B, 820C, 820D (referenced herein collectively or individually as 820). Each of the low reach index optical core nodes 855 connects to a small number of high capacity electronic edge nodes 840. A high reach index optical core node 850 connects to a large number of electronic edge nodes 820, 840.

As in the non-uniform network 500 of FIG. 5, some electronic edge nodes 820 connect to a subset of the optical core nodes 850V, 850X in the optical core 860, while other electronic edge nodes 840 connect to all of the optical core nodes 850, 855 in the optical core 860. Additionally, as in the non-uniform network 500 of FIG. 5, those electronic edge nodes 820, 840 that are connected to the same optical core node 850, 855 can exchange data directly, while electronic edge nodes 820, 840 that are not connected to the same optical core node 850, 855 must exchange data through a tandem electronic edge node 840, twice traversing the optical core 860.

In the networks of FIGS. 2, 5, 7 and 8, a link from a source edge node to a core node is called an uplink and a link from a core node to a sink edge node is called a downlink. A channel in an uplink is called an upstream channel and a channel in a downlink is called a downstream channel as described earlier.

FIG. 9 illustrates a non-uniform network 900 of the same type as the network 800 of FIG. 8, presented in a simplified form. FIG. 9 illustrates an array of edge nodes 920-1, 920-2, 920-3, 920-4, 920-5, 920-6, 920-7, 920-8, 920-9, 920-10, 920-11, 920-12, 920-13, 920-14, 920-15, 920-16, 920-17, 920-18, 920-19, 920-20 (referred to collectively or individually as 920) of different capacities and reach indices, and an array of optical core nodes 950-1, 950-2, 950-3, 950-4, 950-5 (referred to collectively or individually as 950) forming an optical core 960. The edge nodes 920 are arranged according to reach, with those of high reach appearing on the left side. When several edge nodes 920 are of the same reach index, they are arranged consecutively. The arrangement of FIG. 9 is a logical representation of the non-uniform composite star network 900 which clearly illustrates the connectivity of the edge nodes 920 to the optical core nodes 950. It is understood that each line connecting an edge node to a core node represents two links, an uplink and a downlink, which can be of different capacities. With each link carrying multiple wavelength channels, and with each channel having the same capacity, 10 Gb/s for example, it follows that a high capacity link carries a larger number of wavelength channels. The connectivity of network 800 of FIG. 8 ensures that at least one core node has a full-reach, i.e., its reach index equals the number of edge nodes.

This connectivity pattern, when complemented with a method of selecting an optical core node, and switching planes within a selected optical core node, for a particular connection, increases the throughput of a non-uniform optical-core network. When the optical core nodes 950 are operated in a TDM mode, the use of temporal-packing procedures, described below with reference to FIGS. 25 to 28, in assigning time slots for a connection further enhances the throughput and performance of the non-uniform network 900.

In the network of FIG. 9, preferably at least one core node is configured as a full-reach core node which receives at least one input channel from each edge node and originates at least one channel to each edge node as described above. The geographical location of a full-reach optical core node terminating J input links and originating K output links is preferably selected to minimize the aggregate product of link capacity and propagation delay, the aggregate product being defined by the objective function:

$F = \left( {{\sum\limits_{j = 0}^{J - 1}{L_{j}D_{j}}} + {\sum\limits_{k = 0}^{K - 1}{\Lambda_{k}\Delta_{k}}}} \right)$

-   -   where L_(j) is the number of channels in input link j, D_(j) is         the propagation delay along input link j, 0≦j≦J, Λ_(k) is the         number of channels in output link k, Δ_(k) is the propagation         delay along output link j, 0≦k<K.

Numerical methods for minimizing the above objective function are known in the field of system optimization.

Routing

To transfer data from a source edge node (i.e., an edge node connected to a source of data traffic) to a sink edge node (i.e., an edge node connected to a sink of data traffic) in a network such as the non-uniform, composite-star network 800 of FIG. 8, it is desirable that the source node select the optical core node 850, 855 with the least reach index to which it connects. As will be appreciated by a person skilled in the art, a typical edge node includes both a source node and an associated sink node. Additionally, it will be understood that a single-line representation of a link connecting an electronic edge node to an optical core node in FIGS. 2, 5, 7, 8 and 9 may be representative of both an uplink and a downlink, which may carry multiple wavelength channels each. Through the selection, by the source edge nodes, of the least reach index optical core node 850, 855, ports on high reach index optical core nodes 850 tend to remain free for the use of those electronic edge nodes that are not connected to low reach index optical core nodes 855, and therefore have fewer candidate paths to other electronic edge nodes.

Each route from a source edge node to a sink edge node may be classified according to a composite metric that is based on two factors. The first factor is the reach index of the optical core node 850, 855 that is used to connect the source edge node to the sink edge node. The second factor is the propagation delay of the uplink from the source edge node to the optical core node 850, 855 added to the propagation delay of the downlink from the optical core node 850, 855 to the sink edge node to give a propagation delay for the route. Alternatively, the composite metric may be based on different combinations of factors, for instance, available capacity and propagation delay.

As mentioned hereinbefore, the second approach to allow non-uniform capacity of edge nodes implies using non-uniform capacity of the links, i.e., unequal numbers of wavelength channels, between a given optical core node and the edge nodes connected to the given optical core node. FIG. 10 represents a non-uniform optical core node 1000 having an exemplary set of switching planes 1030N, 1030P, 1030Q, 1030R, 1030S, 1030T (referred to collectively or individually as 1030) of different coverage indices connected to edge nodes of different capacities over multi-channel links of different capacities. Notably, switching plane 1030P is asymmetrical, having two input ports and four output ports.

The capacity of a WDM link, whether it is an uplink or a downlink is determined according to the bit rate of the WDM wavelength channels carried. With each wavelength channel modulated at the same rate, 10 Gb/s for example, the capacity of a WDM link is simply determined by the number of wavelength channels carried.

In FIG. 10, the non-uniform optical core node 1000 receives optical signals on high capacity input links 1042A, 1042B at corresponding high capacity demultiplexers 1041A, 1041B and low capacity input links 1022G, 1022H at corresponding low capacity demultiplexers 1021G, 1021H. The non-uniform optical core node 1000 also receives optical signals on medium capacity input links 1032C, 1032D, 1032E, 1032F, at corresponding medium capacity demultiplexers (not shown). The demultiplexers 1041A, 1041B, . . . , 1021G, 1021H demultiplex optical signals received on corresponding input links 1042A, 1042B, . . . , 1022G, 1022H into constituent wavelength channels, which are subsequently transmitted to the switching planes 1030.

The exemplary set of switching planes 1030 of different coverage includes one 2×2 switching plane 1030N, one 2×4 switching plane 1030P, two 4×4 switching planes 1030Q, 1030R and two 8×8 switching planes 1030S, 1030T. The switching planes 1030 are under control of a core node controller 1070. Relatively small sizes of the switching planes 1030 are used in the example of FIG. 10 for clarity. In a typical network, the size of an optical switching plane 1030 would be of the order of 128×128.

As will be apparent to a person skilled in the art, the switching planes 1030 represented in FIG. 10 need not be symmetrical, i.e., have equal numbers of input and output ports. In any switching plane, the number of input ports may not equal the number of output ports. This asymmetry is particularly needed for handling multicast where data from a single input port may be sent to various destination edge nodes through two or more output ports of the optical switching plane. It is contemplated, for example, that there could be switching planes having five input ports and eight output ports or 12 input ports and 14 output ports.

The non-uniform optical core node 1000 transmits optical signals on high capacity output links 1044A, 1044B from corresponding high capacity multiplexers 1043A, 1043B and on low capacity output links 1024G, 1024H from corresponding low capacity multiplexers 1023G, 1023H. The non-uniform optical core node 1000 also transmits optical signals on medium capacity output links 1034C, 1034D, 1034E, 1034F from corresponding medium capacity multiplexers (not shown). The multiplexers 1043A, 1043B, . . . , 1023G, 1023H multiplex wavelength channels received from the switching planes 1030 into optical signal transmitted on corresponding output links 1044A, 1044B, . . . , 1024G, 1024H.

The input side of the non-uniform optical core node 1000 connects to the parallel switching planes 1030 through internal input links 1056, each carrying a single wavelength channel, and the parallel optical switching planes connect to the output side of the non-uniform optical core node 1000 through internal output links 1076, each carrying a single wavelength channel. According to an aspect of the present invention, at least one of the optical switching planes 1030 provides full-coverage, in that at least one wavelength channel from any input link may be switched to any output link.

Network Coverage

In an optical core node comprising a single switching plane, the wavelength channels of each input link are connected to input ports and the wavelength channels of each output link are connected to output ports of the same switching plane. In an optical core node comprising multiple switching planes, the upstream/downstream wavelength channels of each uplink/downlink are connected to input/output ports in different optical switching planes. With each fiber link having W channels, excluding a signaling channel, and with Π denoting the maximum number of input ports or output ports of a symmetrical switching plane, the maximum number, L, of links per core node is L=└Π/W┘ in the single-switching-plane core node, where └r┘ denotes the integer part of a real number r. In a core node using W parallel switching planes (i.e., as many parallel switching planes as there are channels in a link), a maximum of Π links (L=Π) can be supported. Thus, with Π=128, and W=32, a single-switching-plane core node supports only four dual links (a dual link refers to an input link and a output link), while the multiple-switching-plane core node supports 128 dual links. It is noted that an input link to a core node is an uplink from an edge node to the core node and an output link from a core node is a downlink from the core node to an edge node.

The low capacity input link 1022H, originating from a low capacity source edge node, and the corresponding low capacity output link 1024H, destined to a low capacity sink edge node, connect only to the full coverage switching planes 1030S, 1030T. The high capacity input link 1042A, originating from a high capacity source edge node, and the corresponding high capacity output link 1044A, connect to the full coverage switching planes 1030S, 1030T and the partial coverage switching planes 1030N, 1030P, 1030Q, 1030R, i.e., all switching planes 1030. In FIG. 10, a single channel connects a source edge node to a switching plane 1030 and a single channel connects switching plane 1030 to a sink edge node. FIG. 10 is a logical representation and, therefore, omits aspects of the non-uniform optical core node 1000 that are understood to be present.

To establish a route through a core node in a composite star network, such as the network 200 of FIG. 2, an edge node sends a connection request to the core node. Such a connection request would identify a source edge node, a sink edge node and a required capacity for the connection between the edge nodes. The core node, in response to such a connection request, selects a route through a particular switching plane, or through a plurality of switching planes, to satisfy the request and sends an acceptance indication back to the source edge node accepting the connection request. Where the core node does not have the available capacity to satisfy the connection request, the core node sends a refusal indication to the source edge node and any resources reserved during the connection setup attempt are released.

In a non-uniform network with optical core nodes having switching planes of different coverage indices, an exemplary one of which (the non-uniform optical core node 1000) is illustrated in FIG. 10, it is desirable that the route selected to satisfy a connection request use the switching plane(s) 1030 of smallest coverage index that can provide paths for the connection.

In general, the number of channels in an uplink from a given edge node to a core node may differ from the number of channels in a downlink from the core node to the same edge node. This may occur due to traffic asymmetry. In an example of such traffic asymmetry, an edge node may receive substantially more that than it transmits, or vice versa.

The core node controller 1070 includes a processor (not illustrated) with a connection to a memory, which may be used to store data structures described herein. The processor also has interfaces to the switching planes (not illustrated) used for bi-directional communication with the switching planes 1030 (FIG. 10). Controller 1070 exchanges control data with the edge nodes through either dedicated control channels (not illustrated) when channel-switching is used or through time slots in a TDM frame when TDM switching is used. Details of control data exchange between a core node comprising parallel switching planes and electronic edge nodes are described in Applicant's copending U.S. patent application Ser. No. 10/054,509, filed on Nov. 13, 2001 and titled “Time-Coordination in a Burst-Switching Network”. If each of switching planes 1030 receives one channel of the same wavelength (wavelength band) from each input link, then the non-uniform optical core node 1000 does not require wavelength conversion at any of the input or output ports of any switching plane. It may be desirable, however, to allow an input link to connect to a given switching plane through two or more wavelength channels, and/or to allow an output link to connect to the given switching plane through two or more wavelength channels. Since each input link carries channels of different wavelengths (wavelength bands), then connecting two or more channels to a switching plane results in two or more input ports receiving optical signals of different wavelengths. Each output link carries optical signals of different wavelengths. If an output link connects to any given switching plane by two or more channels, or if said output link has only one channel to the given switching plane but any two input ports of the given switching plane connect to channels of different wavelengths (wavelength bands), then wavelength conversion is required to enable any input port to communicate with any output link carrying non-overlapping wavelength channels.

FIG. 11A illustrates the use of wavelength conversion in conjunction with a switching plane 1130 having eight input ports (not shown) and eight output ports (not shown). The eight input ports receive eight wavelength channels on eight internal input links 1156-1, 1156-2, 1156-3, 1156-4, 1156-5, 1156-6, 1156-7, 1156-8 from six input links 1121-1, 1121-2, 1121-3, 1121-4, 1121-5, 1121-6. The eight output ports transmit eight wavelength channels on eight internal output links 1176-1, 1176-2, 1176-3, 1176-4, 1176-5, 1176-6, 1176-7, 1176-8 carried by output links 1191-1, 1191-2, 1191-3, 1191-4, and 1191-5. The eight internal output links 1176 become the five output links 1191 at a MUX stage. Four of the input links 1121-1, 1121-2, 1121-3, 1121-4 provide one wavelength channel each to the switching plane 1130. Two of the input links 1121-5, 1121-6 provide two wavelength channels each to the switching plane 1130. Three of the output links 1191-1, 1191-2, 1191-3 receive one wavelength channel each from the switching plane 1130. One of the output links 1121-4 receives three wavelength channels from the switching plane 1130 and one of the output links 1121-5 receives two wavelength channels from the switching plane 1130. It is noted that each input link 1121 and each output link 1191 may carry several wavelength channels connecting to other switching planes 1130.

To enable any wavelength channel from any input link 1121 to connect to any wavelength channel in any output link 1191, input wavelength converters 1140 can be provided, one for each in the internal input links 1156. The input wavelength converters 1140 act to convert the wavelength of the incoming wavelength channel to the required wavelength of the corresponding outgoing wavelength channel, where the required outgoing wavelength channel is determined through the switching instruction provided to the switching plane 1130. In FIG. 11A, the required wavelength for each of the links 1191-1, 1191-2, and 1191-3 is λ4, the required wavelengths for link 1191-4 are λ4, λ5, and λ6, the required wavelengths for link 1191-5 are λ4 and λ5. Thus, if input link 1121-4 wishes to connect to output link 1191-5, it can do so without wavelength conversion. However, if input link 1121-2 wishes to connect to output link 1191-5, it must shift its wavelength channel from λ4 to λ5 to access link 1191-5 through internal link 1176-8.

If input link 1121-5, which provides two wavelength channels λ4 and λ5 carried on internal links 1156-5 and 1156-6, wishes to connect to output links 1191-1 and, 1191-2, wavelength channel λ5 carried on internal link 1156-6 must be shifted to λ4.

Each of the output links 1191 can carry several wavelength channels from several parallel switching planes 1130 and thee selection of the wavelength channels illustrated in FIG. 11A or 11B, is dictated by the requirement that an output link carry non-overlapping wavelength channels.

Alternatively, wavelength converters may be provided at the output of a switching plane. In FIG. 11B, the elements retain the reference numerals of FIG. 11A. However, the function provided by the input wavelength converters 1140 in FIG. 11A is, instead, provided by output wavelength converters 1150 one each in the internal output links 1176 in FIG. 11B. As in the case of the input wavelength converters 1140, the output wavelength converters 1150 convert to wavelengths based on switching instructions provided to the switching plane 1130.

Practically, it is preferable to deploy identical switching planes in a given optical core node. In an optical core node having N>1 optical switching planes, each having P1 input ports and P2 output ports, with the optical core node supporting input links of different wavelength channel counts and output links of different wavelength channel counts, the degrees of the N switching planes may be equal but the coverage indices of the N switching planes may differ significantly depending on the distribution of the number of channels in the input links and the output links.

FIG. 12 illustrates an alternative arrangement for the non-uniform optical core node 1000 of FIG. 10. In a non-uniform optical core node 1200 presented in FIG. 12, those switching planes 1030N, 1030P, 1030Q, 1030R of FIG. 10 having partial coverage and each carrying the same wavelength-set may be replaced with replacement switching planes 1230NP, 1230QR (referred to herein collectively or individually as 1230) with the same total number of ports. The input-output connectivity of the replacement switching planes 1230 would resemble that of switching plane 1130, of FIG. 11A or FIG. 11B. A core node controller 1270 oversees the operation of the switching planes 1230, 1030. Furthermore, the non-uniform optical core node 1200 retains the multiplexers 1021, 1041 and demultiplexers 1023, 1043 of the non-uniform optical core node 1000 of FIG. 10. Controller 1270 exchanges control data with the edge nodes through either dedicated control channels (not illustrated) when channel-switching is used or through time slots in a TDM frame when TDM switching is used. The basic difference between the original switching planes 1030 and the replacement switching planes 1230 is that each switching plane 1030 has single wavelength channel connections to subtending demultiplexers while a switching plane 1230 may connect to a subtending demultiplexer by more than one wavelength channel.

Selected wavelength channels from high capacity source nodes, included in the input links 1042, are grouped on the replacement switching planes 1230. For example, the 2×2 switching plane 1030N and the 2×4 switching 1030P are replaced with a single 4×6 switching plane 1230NP. Two wavelength channels from the high capacity input link 1042A are grouped on the 4×4 switching plane 1230NP. In this arrangement, high capacity demultiplexer 1041 may have two or more wavelength channels connecting to a replacement switching plane 1230. Without wavelength converters at selected input ports or output ports of the replacement switching planes 1230, switching from input to output is restricted to channels of the same wavelength. Consequently, although the number of ports per switching plane on the replacement switching planes 1230 is higher than on the replaced switching planes 1030, the coverage of the replacement switching plane 1230 and the total coverage of the replaced switching planes 1030 are the same.

If wavelength converters are employed, at the output ports, for example, of the replacement switching planes 1230, then an upstream channel of any wavelength can switch to a downstream channel of any wavelength. This flexibility increases the probability of matching a free upstream channel of an input link 1022, 1032, 1042 to a free downstream channel of an output link 1024, 1034, 1044, due to the removal of the same-wavelength restriction. The probability of mismatch blocking is therefore reduced.

In review, the input and output sides of the non-uniform optical core node 1200 of FIG. 12 are identical to the corresponding sides in the non-uniform optical core node 1000 of FIG. 10. The non-uniform optical core node 1200, however, has fewer optical switching planes 1030/1230, and, consequently, a high capacity input link 1042 must connect to at least one of the optical switching planes 1230 through two or more single-channel internal links 1056. Likewise, a high capacity output link may connect to an optical switching plane 1230 through two or more single-channel internal links 1076. An internal input link 1258 and an internal output link 1278 are illustrated, in FIG. 12, by thick lines to represent internal links including two or more single-channel internal links 1056, 1076. The connectivity in the configuration of the non-uniform optical core node 1200 of FIG. 12 is preferably devised in a manner that maximizes the number of full-coverage switching planes.

FIG. 13 is a plot 1300 showing a desirable frequency distribution of connections among the multiple switching planes of an exemplary optical core node. The switching planes are sorted logically by increasing coverage index along the horizontal axis. It is indeed desirable that a source node S and a sink edge node D try to connect on the switching plane with the lowest coverage index that has an available path, leaving ports on high coverage index switching planes free for the edge nodes that are not connected to low coverage index switching planes. A path selection method, for use within a core node to achieve the desired distribution of FIG. 13, is described below in conjunction with FIGS. 22 to 28.

FIG. 14 illustrates an exemplary input-side vacancy data structure 1400 for representing input-side vacancy of the optical core node 1000, to be used for connection setup in the optical core node 1000. Such an input-side vacancy data structure 1400 may be maintained at each optical core node. An element of the input-side vacancy data structure 1400 is denoted u(q,j), where q is a switching plane index and j is an input-port index and. Each entry in data structure 1400 has a value of 0 or 1 (not illustrated) to indicate whether a corresponding input port is free or already assigned to a channel from an input link. The entries shown indicate the input link indices and are provided in the figure for illustration only. It is noted also that the structure 1400 can be used only for optical core nodes of the type of the non-uniform optical core node 1000 of FIG. 10, wherein each switching plane 1030 receives only one channel from any input link. A data structure that can be used for either the optical core node 1000 of FIG. 10 or the optical core node 1200 of FIG. 12 is described below with reference to FIGS. 20 and 21.

The exemplary input-side vacancy data structure 1400 relates to the non-uniform optical core node 1000 of FIG. 10, which comprises six parallel switching planes 1030. In FIG. 14, the six parallel switching planes 1030 are sorted logically by coverage index and indexed on the vertical axis. The optical core node 1000 receives data from eight source edge nodes through input links 1022, 1032, 1042 of various capacities. The input ports of the six parallel switching planes 1030 are indexed from 0 to 7 on the horizontal axis. It is assumed, in this example, that each channel in a given input link 1022, 1032, 1042 connects to the same source edge node and that a distinct source edge node connects to each input link. It is possible, however, to have several input links connecting to the same source edge node. On the input side of the optical core node 1000, the 2×2 switching plane 1030N and the 2×4 switching plane 1030P are connected to the two high capacity input links 1042, the two 4×4 switching planes 1030Q, 1030R are connected to four input links 1042, 1032 and the two 8×8 switching planes 1030S, 1030T are connected to eight input links 1022, 1032, 1042.

Each entry in the input-side vacancy data structure 1400 corresponds to an input port on a switching plane 1030. The value of the entry indicates whether the port is used for a connection. For instance, the value of an entry may be 1 if the port is vacant and 0 if the port is occupied. Each row in the input side vacancy data structure 1400 contains information about the connections to the switching plane 1030 having the switching-plane index associated with that row. Likewise, each column in the input side vacancy data structure 1400 contains information about the input link 1022, 1032, 1042 having the input-port index associated with that column.

FIG. 15 illustrates an exemplary output-side vacancy data structure 1500 for representing the output side vacancy of the non-uniform optical core node 1000 whose input-side vacancy is represented in FIG. 14. The output-side vacancy data structure 1500 may be used for connection setup in the non-uniform optical core node 1000. Such an output-side vacancy data structure 1500 may be maintained at each optical core node. An element in output side vacancy data structure 1500 is denoted v(q,k), where q is a switching plane index and k is an output-port index.

On the output side of the optical core node, the 2×2 switching plane 1030N is connected to the two high capacity output links 1044, the 2×4 switching 1030P and the two 4×4 switching planes 1030Q, 1030R are connected to four output links 1044, 1034 and the two 8×8 switching planes 1030S, 1030T are connected to eight output links 1024, 1034, 1044.

An input port is identified by its switching-plane index, and input-port index within its switching plane. Likewise, an output port is identified by its switching-plane index, and output-port index within its switching plane.

FIG. 16 illustrates an alternative connectivity where the input-side connectivity of the configuration of the optical core node 1000 of FIG. 10 is replaced by the input-side connectivity of the configuration of the optical core node 1200 of FIG. 12. FIG. 16 is derived from FIG. 14 by replacing the connections to input ports (4, 0), (5, 0), (4, 1) and (5, 1) with connections to input ports (3, 4), (2, 4), (3, 5), and (2, 5) respectively.

FIG. 17 illustrates an alternative connectivity where the output-side connectivity of configuration 1000 is replaced by the output-side connectivity of configuration 1200. FIG. 17 is derived from FIG. 15 by replacing the connections to output ports (4, 0), (5, 0), (4, 1), (5, 1), (4, 2), and (4, 3) with connections to output ports (3, 4), (2, 4), (3, 5), (2, 5), (3, 6) and (2, 6), respectively.

FIG. 18 illustrates a highest-index-plane data structure 1800 to be used for connection setup in the non-uniform optical core node 1000 of FIG. 10. Such a highest-index-plane data structure 1800 may be maintained at each optical core node. The highest-index-plane data structure 1800 contains information about the presence of input links to the switching planes 1030. Each column indicates the index of the highest-index switching plane 1030 to which an input link 1022, 1032, 1042 or an output link 1024, 1034, 1044 connects. The switching plane index is the index associated with the switching planes in FIG. 14 and FIG. 15. Input links 1022, 1032, 1042 and output links 1024, 1034, 1044 are indexed from 0 to 7 on the horizontal axis. The highest-index plane of each input link 1022, 1032, 1042 is listed in a top row 1802 of the highest-index-plane data structure 1800 while the highest-index-plane of each output link 1024, 1034, 1044 is listed in a bottom row 1804 of the highest-index-plane data structure 1800. An element in the top row 1802 of the highest-index-plane data structure 1800 is denoted P(j), where j is an input link index. An element in the bottom row 1804 of the highest-index-plane data structure 1800 is denoted Q(k), where k is a sink node index.

With reference to FIG. 10, input link G 1022G (input port index=6) connects to the 8×8 switching planes 1030S, 1030T. With reference to FIG. 14, one 8×8 switching plane 1030S has a switching-plane index of 1 and while the other 8×8 switching plane 1030T has a switching-plane index of 0. Therefore, the highest-index switching plane to which input link G 1022G connects is the switching plane 1030S having index of 1. This result can be read directly from FIG. 14. The highest-index switching plane 1030 connecting from the source node G 1022G (which has an input port index of 6) has a switching-plane index of 1, so P(6)=1. Similarly, the highest-index switching plane 1030 connecting to output link 1044B (output port index=1) has a switching-plane index of 5, so Q(1)=5.

FIG. 19 illustrates a highest-index-plane data structure 1900, similar to that of structure 1800, to be used for connection setup in the non-uniform optical core node 1200 of FIG. 12.

FIG. 20 illustrates an input-side connectivity description. A connectivity matrix 2000 has N rows, each corresponding to a switching plane and (J+1) columns, J being the number of input links. An array 2020 contains identifiers of the P1 input ports. An input-port identifier is preferably an input-port number, 0 to P1−1. An entry ξ(m, j) of input-side connectivity matrix 2000 points to the identifier, in the array 2020, of the first input-port in switching plane m to which input link j connects. The number of input ports in a switching plane connecting to channels of an input link j is determined as n1=ξ(m, j+1)−ξ(m, j), 0≦j<J, and the identifiers of the n1 input ports, when n1 is greater than zero, are found in the array 2020 at indices ξ(m, j) to ξ(m, j+1)−1.

For example, input link 1, which has six channels, has one channel connected to switching plane 0, one channel connected to switching plane 1, two channels connecting to switching plane 2 and two channels connecting to switching-plane 3 as indicated in FIG. 17. These can be determined from the connectivity matrix 2000 by noting that, for switching plane 0, ξ(0, 1)=21, ξ(0, 2)=22 and 22−21=1. Additionally, for switching plane 1, ξ(1, 1)=13, ξ(1, 2)=14 and 14−13=1. For switching plane 2, ξ(2, 1)=8, ξ(2, 2)=10 and 10−8=2. For switching plane 3, ξ(1)=2, 86 (3, 2)=4 and 4−2=2. The connecting input port for switching plane 0 is identified in the array 2020 at index ξ(0, 1)=21 as 1. The connecting input port for switching plane 1 is identified in the array 2020 at ξ(1, 1)=13 as 1. The two connecting input ports for switching plane 2 are identified in the array 2020 starting at ξ(2, 1)=8 as 1 and 5. The two connecting input ports for switching plane 3 are identified in the array 2020 starting at ξ(3, 1)=2 as 1 and 5. These can be verified from FIG. 16.

FIG. 21 illustrates an output-side connectivity description. A connectivity matrix 2100 has N rows, each corresponding to a switching plane and (K+1) columns, K being the number of output links. An array 2120 contains identifiers of the P2 output ports. An output-port identifier is preferably an output-port number, 0 to P2−1. An entry η(m, k) of output-side connectivity matrix 2100 points to the identifier, in the array 2120, of the first output port in switching plane m connecting to output link k. The number of output ports in a switching plane connecting to channels of an output link k is determined as n2=η(m, k+1)−η(m, k), 0<k<K, and the identifiers of the n2 input ports, when n2 is greater than zero, are found in array 2120 at indices η(m, k) to η(m, k+1)−1.

For example, where output link 2, which has five channels, has one channel connecting to output port 2 of switching plane 0, one channel connecting to output port 2 of switching plane 1, one channel connecting to output port 2 of switching plane 2, and two channels connecting to output ports 2 and 6 of switching plane 3. This connectivity can be determined directly from the matrix 2100 and the array 2120 of FIG. 21. For example, η(3, 2)=4, η(3, 3)=6, indicating that output link 2 connects to two (6−4=2) output ports identified in entries 4 and 5 in array 2120. Entries 4 and 5 indicate that the output-port identifiers are 2 and 6. This result can be read from matrix 1700 of FIG. 17.

The connectivity of each input link and each output link are clearly illustrated in the data structures of FIG. 16 and FIG. 17, respectively. However, for hardware or software mechanization, the structures of FIG. 20 and FIG. 21 may be used. Within a switching plane m, 0≦m<N, a connection from input link j to output link k, 0≦j<J, and 0≦k<K, may be established from any of the n1 input ports indexed in array 2020 from ξ(m, j) to ξ(m, j+1)−1 to any of the n2 output ports indexed in array 2120 from η (m, k) to η (m, k+1)−1.

FIG. 22 is a flow chart of the connection setup procedure 2200 in the non-uniform optical core node 1200 of FIG. 12. The method illustrated in FIG. 22 may be executed by the core node controller 1270 of the non-uniform optical core node 1200 and makes use of the data structures of FIGS. 16 to 21. The procedure is activated upon reception of a connection request from a source edge node (step 2202) through an input link 1022, 1032, 1042 of index j. The index, k, of the output link 1024, 1034, 1044 and an indication, Y, of a requested number of channels for the requested connection are determined from the information provided in the connection request. The connection request may specify, for example, a bit rate for the connection rather than the number of channels. The bit rate is then converted into a number, Y, of channels.

The vacancy, i.e., available capacity in terms of the number of unassigned channels, of the output link 1024, 1034, 1044, to a sink edge node may be maintained in an array (not shown) in the core node controller 1270 and an element of the array is denoted Ω(k). In step 2204, this array is consulted to determine the vacancy of the output link(s) 1024, 1034, 1044 to the sink edge node specified in the connection request. It is then determined whether the vacancy, Ω(k), of the output links 1024, 1034, 1044 to the specified sink edge node equals or exceeds the requested capacity, Y (step 2206). A rejection message is produced (step 2218) and sent to the requesting source edge node when it is determined that the requested capacity exceeds the vacancy of the output link 1024, 1034, 1044 (step 2206).

If the vacancy equals or exceeds the requested capacity, an initial value for a switching plane index, q, may then be determined as the lower of P(j) and Q(k) (step 2208), given the received j and k. Recall that P(j) is read from array 1802 of FIG. 19 and Q(k) is read from array 1804 of FIG. 19, and that an entry u(., .) or v(., .) in vacancy data structures 1600 or 1700 (and an entry in data structure 2400) contains either a ‘0’ or a ‘1’ to indicate whether a corresponding input port or output port is free or occupied. The switching plane having index q may be called a “candidate” switching plane. Specifically, values u(q, j) and v(q, k) are retrieved, where u(q, j) is the entry in the input side vacancy data structure 1600 corresponding to the input port, of the candidate switching plane, that is connected to the output link 1022, 1034, 1042 having index j and v(q, k) is the entry in the output side vacancy data structure 1700 corresponding to the output port, of the candidate switching plane, that is connected to the output link 1024, 1034, 1044 having index k.

Subsequently, in step 2210, a procedure for allocating Y channels through switching planes indexed from q to 0 is executed. The procedure determines a number of allocable channels not exceeding the requested number Y. If it is determined (step 2212) that the procedure of step 2210 has successfully found Y allocable channels for the requested connection, then the corresponding element Ω(k) and elements u(m, j), v(m, j), over the selected switching planes, m, are updated (step 2214). A message is then sent to the requesting source node (step 2216) indicating that the connection request is accepted and indicating the allocable channels corresponding to the selected switching planes. If the procedure of step 2210 fails to allocate the required Y channels, it is determined that a route is not available and a rejection message is produced (step 2218) and sent to the source edge node. Upon connection termination, respective elements u(,), v(,), and Ω( ), must be reset to indicate vacancy states.

FIG. 23 details the channel-allocation process of step 2210 of FIG. 22. The number of requested channels for the connection request is denoted Y. A variable Φ denoting the remaining number of channels to be allocated is initialized in step 2312 to be equal to Y. In step 2314, the switching plane of index q, determined in step 2208 is selected, by setting an intermediate switching-plane index m to be equal to q. In step 2320, the number, N1, of input-side free ports in switching plane m is determined and the N1 free input ports are identified using the data structure of FIG. 20. Likewise, in step 2324, the number, N2, of output-side free ports in switching plane m is determined and the N2 free input ports are identified using the data structure of FIG. 21. In step 2328, the least of N1, N2, and Φ is determined. The resulting number of allocable channels is denoted n. In step 2330, the remaining number of channels, Φ, is reduced by the value of n. In step 2334, if the value of Φ is zero, the channel-assignment process is successful and the identities of the allocated input ports and output ports are reported for the benefit of step 2214 (FIG. 22). Process control is then returned to the process of FIG. 22.

If the value of Φ is greater than zero, the switching-plane index m is examined. If the index, m, is determined (step 2338) to be greater than zero, the index is reduced by one so that the next switching plane may be selected and the procedure starting with step 2320 may be repeated. Where the index, m, which is the index of the last switching plane considered, is determined to be zero (step 2338), it is determined that there is insufficient vacancy in all the switching planes 0 to q. Subsequent to the consideration that there is insufficient vacancy, the channel-allocation process is reported as unsuccessful (step 2342) and process control is returned to the process of FIG. 22.

In an alternative procedure, the cumulative number of available free channels in all the considered switching planes is determined first before actually identifying the respective input ports and output ports. However, in a properly provisioned network, the probability of connection-request rejection would be minute and it would be more expedient to actually allocate ports while they are counted.

It is noted that, even when step 2206 indicates the availability of a sufficient number of free channels in respective output link, connection blocking can still occur due to spatial mismatch of the channels between the input side and the output side. The availability of a sufficient number of free channels in the respective input link is ascertained by the requesting source edge node before a connection request is made. When step 2218 is reached, entries in Φ(.), u(.,.), and v(.,.) corresponding to a blocked connection attempt must be reset to their vacancy states.

In the non-uniform optical core node 1200 of FIG. 12, the smallest connection-capacity that can be established between two edge nodes is an entire channel. Where an input link 1022, 1032, 1042 to the non-uniform optical core node 1200 is an optical fiber link, a channel could be, for example, a single wavelength modulated at 10 Gb/s. With spatial traffic imbalance, some connections may require a capacity of more than one channel while others would require a capacity that is a small fraction of the capacity of a channel. Thus, a connection setup between a given input link 1022, 1032, 1042 and a given output link 1024, 1034, 1044 where the connection requires significantly less than the capacity of an entire channel would either be allocated a full channel or sent to an intermediate edge node where it would be aggregated with other connections towards the desired sink edge node. It is desirable to allow a finer allocation of capacity through the non-uniform optical core node 1200. Such a finer allocation of capacity may increase the reach of source edge nodes, permitting connections of minute capacity to some sink edge nodes and connections of large capacity to other sink edge nodes simultaneously. This finer allocation of bandwidth can be achieved with Time Division Multiplexing (TDM).

If low switching-latency can be realized in the switching fabric, a time-sharing scheme, such as TDM can be used, and control signals sent by subtending edge nodes can be carried “in-band”, occupying pre-assigned time slots in a TDM frame. In-band signaling in a network deploying fast optical switches in the core is described in Copending U.S. patent application Ser. No. 10/054,509, referenced hereinbefore.

In Time Division Multiplexing, a TDM frame having a fixed duration is divided into a number of time slots. For example, a TDM frame having one millisecond duration may be divided into 256 time slots of 4 microseconds duration each. Connections in a switching plane 1030/1230 can be re-arranged in every time slot in a TDM frame according to a schedule. For example, an input port in a switching plane that receives from a source node “A” may be connected to an output port in the switching plane that sends to a sink node “B” for the duration of one time slot and may be connected to an output port in the switching plane that sends to a sink node “E” in the next sequential time slot in the TDM frame. The schedule may be modified to increase or decrease the number of time slots allocated to a connection between a source node and a sink node to reflect a requested capacity. Each source node can then reach thousands of sink nodes with vastly different capacity allocations. If a link has 32 wavelength channels, for example, with each channel divided into 256 time slots, then the total number of time slots is 8,192. These time slots can be routed independently to an arbitrary number of sink nodes.

The duration of a time slot in a TDM frame is constrained by the switching latency of the optical switch. Low switching latency permits a short time-slot duration. The number of time slots per TDM frame is constrained by an upper bound of the period of the TDM frame. A large period, of 10 milliseconds for example, may result in an unacceptable waiting time at a source edge node. A reasonable frame period would be one millisecond.

The use of TDM in a network having bufferless optical core nodes requires precise time coordination between the edge nodes and the optical core nodes. TDM frames must be aligned in time, within a certain degree of accuracy, at the optical core node so that data in specific time slots in upstream channels can be switched appropriately to intended time slots in downstream channels. This alignment can easily be achieved between electronic nodes having buffers. However, it is impractical to time align a succession of adjacent bufferless optical nodes. Where the non-uniform network 500 of FIG. 5 is adapted for use of TDM, edge-to-edge connections only encounter a single intermediate optical core node between two electronic nodes. The use of single intermediate optical node makes time locking, and therefore TDM in a bufferless path, feasible. TDM also requires fast-switching optical core nodes. A fast-switching optical core node can allocate time slots that are sufficiently short to enable the use of a TDM frame of a relatively short period and avoid undue delay at the source nodes. A switching latency of 100 nanoseconds, for example, permits the use of time slots of one microsecond duration each and a TDM frame of one millisecond would contain 1,000 time slots.

It should be noted that, where at least one of the 8×8 switching planes 1030S, 1030T operates in TDM switching mode, the TDM-operated switching plane may handle signaling and control messages exchanged between edge nodes and the core node controller 1270 in addition to switching payload data from input ports to output ports.

FIG. 24 illustrates a set 2400 of S vacancy data structures 2402-0, 2402-1, . . . , 2402-S−1, 2402-(S−1) for use with a route selection method adapted from the method presented in FIG. 22. It should be understood that sets of input side vacancy data structures and sets of output side vacancy data structures will have a similar logical representation. The sets may, however, be asymmetrical when the switching planes are asymmetrical. Each vacancy data structure 2402 of the set 2400 corresponds to a single time slot in a TDM frame. The route selection method of FIG. 22 and FIG. 23 may be adapted to be applied to successive time slots. This approach realizes a method of route selection that packs connections both in time and in space. Specifically, connections are packed in time by attempting to first allocate available time slots early in the TDM frame. Connections can be packed in space by considering switching planes in a given order. In a non-uniform core node 1200, switching planes of low coverage index for available connections before switching planes of high coverage index, as discussed hereinbefore.

It is noted that the two-dimensional packing process, in time and space, is beneficial for both uniform and non-uniform parallel-plane core nodes. Two-dimensional packing, however, is more crucial in non-uniform core nodes where switching planes of low coverage index are considered first. The throughput and performance of non-uniform networks are sensitive to temporal and spatial traffic variations and; therefore, benefit more from the packing procedure of FIG. 23. In a uniform parallel-plane core node, spatial packing is realized simply by considering, in a connection setup, the switching planes in a prescribed order.

FIG. 25 is a flow chart of the connection setup procedure 2500 where the non-uniform optical core node 1200 uses TDM and it is decided to pack connections in time then in space. The method illustrated in FIG. 25 may be executed by the core node controller 1270 at the non-uniform optical core node 1200 and makes use of the data structures corresponding to the data structures of FIGS. 16 and 17 but having the format illustrated in FIG. 24. Data structure 2400 of FIG. 24 is derived from data structure 1600 of FIG. 16 which applies to the input side of a core node 1200. The same structure 2400 is applied to the output side of the core node 1200 by using an output-side vacancy table 1700 for each time slot in a TDM frame. Notably, the data structures of FIGS. 18 to 21 are unchanged with the use of TDM. The procedure is activated upon reception of a connection request from a source edge node (step 2502) through an input link 1022, 1032, 1042. The index, j, of the of the input link 1022, 1032, 1042 is noted and an index, k, of an output link 1024, 1034, 1044 leading to the desired sink edge node is also noted. An indication of a requested capacity for the requested connection is used to determine the number, Ψ, of requested time slots per TDM frame.

The available capacity, i.e., vacancy, of an output link 1024, 1034, 1044 to a sink edge node may be maintained in a data structure in the memory of the core node controller 1270. An element of the data structure is denoted W(k). In step 2504, the element W(k) is retrieved to determine the vacancy of the output link 1024, 1034, 1044 to the sink edge node specified in the connection request. It is then determined whether the vacancy, W(k), equals or exceeds the requested capacity, Ψ (step 2506), where Ψ is expressed as a number of time slots per TDM frame. Where vacancy equals or exceeds the requested capacity, a time slot index, t, is initialized (step 2508). An initial value for the switching plane index, q, may then be determined as the lower of P(j) and Q(k) (step 2514), given the received j and k. P(j) is the highest-index switching plane for input link j and Q(k) is the highest index switching plane for output link k as determined from arrays 1802 and 1804 of FIG. 29. The switching plane having index q is the first candidate switching plane.

A variable Φ denoting the remaining number of time slots to be allocated is initialized to be equal to Ψ (step 2516). Ports in the switching planes q to 0 that are available for the time slot, t, under consideration, are then allocated (step 2520). Step 2520 determines a number of allocable paths during a given time slot in the TDM frame and is detailed in FIG. 26. The ports allocated in step 2520 are then used to form route descriptions (step 2522) for the time slot, t, under consideration. Where the remaining number, Φ of time slots to be allocated is determined to be zero (step 2524), an acceptance message is sent to the source edge node (step 2526). Such an acceptance message identifies time slots and wavelength channels, of the input links, corresponding to the input ports allocated to satisfy the connection request.

Where the remaining number, Φ, of time slots to be allocated is determined to be greater than zero (step 2524), the index t of the time slot under consideration is increased (step 2528) and, in step 2534, if t=S, where S is the number of time slots per TDM frame (the time slots of a TDM frame are numbered 0 to S−1), it is determined that there are no more time slots to consider and a rejection message may be produced and sent (step 2532) to the source edge node.

A rejection message is produced and sent (step 2532) to the source edge node when it is determined that the output link to sink edge node does not have the capacity to meet the request (step 2506) or, later on, if it is determined that fewer matching time slots than the requested number of time slots are available for allocation (step 2520) after all time slots have been considered (step 2534). If all time slots have not been considered, the port allocation procedure of step 2520 is repeated for the next time slot. When step 2532 is reached, any allocated ports are released by updating the vacancy structure 2400.

FIG. 26 details the time-slot assignment process of step 2520. The procedure is quite similar to that of FIG. 23. The main difference is that the requested capacity is expressed as a number of time slots per TDM frame and, because it cannot be determined whether a requested number Ψ of time slots can be accommodated before repeating procedure 2520 of FIG. 26 for all the time slots of a TDM frame, an acceptance, or otherwise, of a connection requested is determined in step 2534 in the parent procedure 2500.

The integer variable Φ, used to denote the number of time slots still to be allocated, is an input to procedure 2520 of FIG. 26. In step 2516, this number is initialized to equal the required number Ψ of time slots per TDM frame. In step 2614, the switching plane q is selected, by setting the index m equal to q. In steps 2620 and 2624, the N1 input-side ports and N2 output-side ports are identified (these are analogous to steps 2320 and 2324). In step 2628, the minimum (n) of N1, N2 and Φ is determined as the number of allocable paths within a current switching plane m and in step 2630, the integer variable Φ is reduced by the result, n. In step 2634, the procedure is terminated successfully if Φ=0, indicating that the requested number of time slots for the requested connection has been found. If the remaining number of time slots to be allocated, Φ, is determined to be greater than zero (step 2634), it is then determined whether the last switching plane considered was the last switching plane available for consideration, i.e., whether m=0 (step 2638). If there remain switching planes to consider, the switching plane index, m, is reduced by one (step 2640) and the procedure starting with step 2620 is repeated.

The identities of ports allocated to the connection request, if any, are reported (step 2650) whether the reporting step is reached when port allocation has completed successfully (step 2634) or unsuccessfully (from step 2638). Note that the parent procedure 2500 is aware of the index of the time slot for which procedure 2520 is executed.

It is noted that, even when step 2506 indicates the availability of a sufficient number of free time slots per TDM frame in the channels of the respective output link, connection blocking can still occur due to spatial and temporal mismatch of the available time slots between the input side and the output side of the non-uniform optical core node 1200 (FIG. 12). The availability of a sufficient number of free time slots in the respective input link is of course ascertained by the requesting source edge node before a connection request is made.

FIG. 27 is a flow chart of the connection setup procedure 2700 where the non-uniform optical core node 1200 uses TDM and it is decided to pack connections in space then in time, in contrast to packing connections in time then in space as illustrated in FIG. 25. The procedure is activated upon reception of a connection request from a source edge node through an input link 1022, 1032, 1042 (step 2702). The index, j, of the input link 1022, 1032, 1042, and an index, k, of an output link 1024, 1034, 1044 are associated with the request. A required number, Ψ, of time slots per TDM frame is deduced from an indication of required capacity for the requested connection.

The vacancy of the output links 1024, 1034, 1044 to sink node specified in the connection request is then determined (step 2704). It is then determined whether the vacancy, W(k), of the specified output link 1024, 1034, 1044 equals or exceeds the requested capacity, Ψ (step 2706). If the vacancy is lower than the required number of time slots Ψ, a rejection message is sent to the source edge node (step 2732) and the procedure 2700 is complete.

It is noted that, even when step 2706 indicates the availability of a sufficient number of free time slots per TDM frame in the channels of the respective output link, connection blocking can still occur due to spatial and temporal mismatch of the free time slots between the input side and the output side. The availability of a sufficient number of free time slots in the respective input link is of course ascertained by the requesting source edge node before a connection request is made.

If the vacancy of the requested output link is greater than the required number of time slots Ψ, an initial value for the switching plane index, q, may then be determined as the lower of P(j) and Q(k) (step 2710), given the received input-link index j and output-link index k. The switching plane having index q is the first candidate switching plane. An integer variable, Φ, is used to denote the number of time slots still to be assigned for the requested connection. In step 2712, this integer variable Φ is initialized to equal Ψ. Step 2720 is detailed in the procedure of FIG. 28, which is executed to find a number of allocable ports (hence allocable paths), in the switching plan under consideration. The procedure 2720 of FIG. 28 returns to step 2722 the identities of allocated ports in particular time slots, if any, in switching plane m and the remaining number, Φ, of time slots to be allocated. If it is determined that there are no more time slots to be allocated (step 2724), an acceptance message is sent to the source edge node (step 2726). The acceptance message may include identities for the assigned time slots and the wavelength channels associated with respective allocated input and output ports.

If it is determined that there are more time slots to be allocated (from step 2724), i.e., if Φ is not yet zero, then it is determined whether the most recent switching plane considered was the switching plane with the least index (step 2716). If all the switching planes are exhausted without accumulating the required number of time slots for the requested connection, i.e., m=0 (step 2716), a rejection message is sent to the source edge node (step 2732) and the procedure 2700 is complete. However, if there are switching planes left to consider, the switching plane index is reduced by one (step 2718) and the port allocation procedure (step 2720) is repeated for the switching plane of the new index. Step 2732 also includes a process of updating vacancy arrays 2400 if any ports have been allocated.

In FIG. 28, the port allocation procedure begins with the initialization of a time slot index, t, to zero (step 2814). The input ports of the switching plane under consideration are determined for the time slot, t (step 2820). Additionally, the output ports of the switching plane under consideration are determined for the time slot, t (step 2824). In step 2828, the minimum (n) of N1, N2 and Φ is determined as the number of allocable paths during a current time slot ‘t’ of the TDM frame. The remaining number of time slots, Φ, is then reduced by this minimum (step 2830). It is then determined whether the remaining number of time slots, Φ, has been reduced to zero (2834). If Φ has been reduced to zero, the allocated ports and time slots are reported (step 2850) and process control returns to the procedure of FIG. 27. If Φ is greater than zero, the time slot index is increased (step 2838). If, in step 2840, it is determined that the new time-slot index t is less than S, port allocation is considered for this time slot (step 2820), otherwise, the allocated ports and time slots are reported (step 2850) and process control returns to the procedure of FIG. 27.

In review, the method represented by FIG. 25 is used when it is considered more desirable to pack connections in time then in space. The same time slot is tested for availability in successive switching planes before considering the next time slot. In contrast, the method represented by FIG. 27 is used when it is considered more desirable to pack connections in space then in time. Successive time slots in the same switching plane are tested for availability before considering the next switching plane.

Input-Side and Output-Side Connectivity

FIG. 29 is a flow chart of a procedure for assigning the channels of input links to the input ports of an optical core node having parallel switching planes. The same procedure can be applied for assigning the channels of output links to the output ports of the parallel switching planes.

In its general form, the method of assignment of channels to ports, in accordance with the present invention, applies to the assignment of channels arranged in M>1 channel groups to ports arranged into N>1 rows, where each row has a specified number of ports and each port is assigned only one channel.

In a node comprising parallel switching planes, the N rows can represent N switching planes. The ports of a given switching plane can be further divided into input ports and output ports, and the number of input ports may differ from the number of output ports. Each of the M channel groups can be separated into J input groups and (M−J) output groups, where 0<J<M. Each of the J input groups can represent an input link comprising a number of channels, and each of the (M−J) output groups can represent an output link comprising a number of channels. A link is preferably a fiber-optic link carrying wavelength channels.

For ease of presentation, the switching planes are considered to be identical, each having P1 input ports and P2 output ports. The number of input links is denoted J and the number of output links is denoted K. The number of channels in input link j, 0≦j<J, is denoted L(j), and the number of channels in output link k, 0≦k<K, is denoted Λ(k).

In step 2910, the input links are sorted, in a descending order, according to the number of channels per input link, so that L(j)≧L(j+1), 0≧j<J. The input links are, therefore, numbered accordingly. In step 2912, the required number of switching plane is determined. The number, N1, of switching planes required to accommodate the input links is determined as the sum of L(j), 0≦j<J, divided by the number P1 of input ports per switching plane. The number, N2, of switching planes required to accommodate the output links is determined as the sum of Λ(k), 0≦k<K, divided by the number P2 of output ports per switching plane. Thus:

${{N1} = {\frac{1}{P1}{\sum\limits_{j = 0}^{J - 1}{L(j)}}}},{{{and}\mspace{14mu}{N2}} = {\frac{1}{P2}{\sum\limits_{k = 0}^{K - 1}{\Lambda(k)}}}},$ where L(j), 0≦j<J is the number of channels in an input link j, and Λ(k), 0≦k<K is the number of channels in an output link k.

The number, N, of switching planes to be used is determined (step 2912) to be an integer at least equal to the larger of N1 and N2. The N switching planes are numbered 0 to (N−1). A switching plane index, m, is then initialized to zero (step 2914).

An allocation matrix G is used to maintain a record of allocations of channels of input links to input ports. An element g(p, m), of matrix G, 0≦p<P1, 0≦m<N, contains an identifier of an input link that has a channel connected to input port p of switching plane m in a core node having N switching planes with each plane having P1 input ports.

An allocation matrix Γ is used to keep a record of allocations of channels of output links to output ports. An element γ(π, m), of matrix Γ, 0≦π<P2, 0≦m<N, contains an identifier of an input link that has a channel connected to input port π of switching plane m in a core node having N switching planes with each plane having P2 output ports. Both allocation matrices, G and Γ, are initialized by assigning a null entry to each element.

The steps collectively referenced as 2920, including steps 2922 to 2934, are used to assign upstream channels to any selected switching plane. An array L(.), initially indicates the number of channels for each of the J input links. The array L(.) is gradually updated to indicate the number of unassigned channels for each of the input links. In step 2922, the link index, j, is initialized to zero so that the first of the J sorted input links may be considered. In step 2924, it is determined whether there are unassigned channels in input link j. If L(j) is greater than zero, a channel from input link j is allocated to the input port number p=j of switching plane m (step 2926).

Thus, when a channel from input link j is allocated to an input port of switching plane m, the corresponding element of Matrix G, i.e., element g(p, m), p=j, is set equal to j and L(j) is reduced by one. Note that the P1 input ports of any switching plane are numbered 0 to (P1−1).

In step 2928, the link index, j, is increased. In step 2930, it is determined whether increasing the link index has caused j to equal the value J, indicating that all input links have been considered regarding channel-assignment to switching plane m. If j=J, then step 2932 is selected. Where it is determined that j is not equal to J, it is determined whether the input link corresponding to the new link index has channels left to allocate to input ports (step 2924).

An array α(m), 0≦m<N, points to the lowest index input port in switching plane m that is not yet assigned. Thus, initially α(m) is set equal to zero for 0≦m<N.

If, in step 2924, it is determined that no more channels are left to be allocated for the input link j, i.e., L(j) is not greater than zero, or, in step 2930, it is determined that there are no more input links to consider, i.e., j is equal to J, then α(m) is set equal to the last known value of j, which is one more than the last used value of j (step 2932). The maximum value of α(m) is, therefore, P1, because the number, J, of input links is limited to be less than or equal P1. Thus, α(m) points to the lowest index input port in switching plane m that is not yet assigned.

In step 2940, the index, m, of the last considered switching plane is examined. If it is determined that m corresponds to the last switching plane, N−1, the initial assignment process of collective step 2920 is determined to be complete and a subsequent assignment process is initiated. Where the initial assignment process of collective step 2920 is determined to be incomplete, i.e., m is less than N−1, the switching plane index is increased (step 2934) and the initial assignment process is repeated for a new switching plane.

In the subsequent assignment process, the unassigned channels in a subset of the input links are distributed among the switching planes, starting with switching plane (N−1), in a manner that attempts to increase connectivity. Since the unassigned channels are to be found in a subset of input links having consecutive numbers starting with j=0, the link index, j, is re-initialized to zero (step 2970). In step 2972, the number of unassigned channels in input link j, L(j), is considered. If the number of unassigned channels in input link j is positive, the value of a variable ‘a’ is set to the value of α(m) (step 2974). It is then determined whether the variable ‘a’ is equal to the number of input ports, P1 (step 2976). If all the input ports switching plane m are assigned, i.e., a =α(m)=P1, the switching plane index is set equal to the highest switching-plane index (N−1) (step 2986) and the value of the variable ‘a’ is set to the value of α(m) (step 2974) for the new switching plane m. If some of the input ports of switching plane m are unassigned, i.e., a =α(m)<P1, the entry g(a, m) is updated (step 2978), replacing a null entry with the input link index, j, the number of unassigned channels L(j) is reduced by one and the input port pointer α(m) is increased by one. If the last processed switching plane is determined to be the bottom switching plane (m=0) (step 2980), then the switching plane index is set to the number of switching planes, N (step 2984). In step 2982, the switching plane index is reduced by one and the link index is increased by one, modulo J. Subsequently, the number of unassigned channels in the new input link j, L(j), is considered (step 2972). Step 2982 may also be reached when it is determined that the last processed switching plane is other than the bottom switching plane (step 2980).

In step 2988, if it is found that the input link that has all its channels assigned is not input link number 0 (j=0), then the link index is re-initialized (step 2970). Where the input link that has all its channels assigned is the input link number 0, the channel-assignment procedure is considered to be complete because input link number 0 has the highest number of channels and the assignment process described above ensured that input link number 0 is the last input link to be exhausted.

The procedure of FIG. 29 is described herein as the assignment of wavelength channels in input links to input ports of switching planes. The procedure for the assignment of wavelength channels in output links to output ports of switching planes is identical, with array L(.) replaced by array Λ(.), and the number of input ports P1 replaced by the number of output ports P2.

FIG. 30 illustrates an input-side connectivity matrix G 3000 for an exemplary parallel-plane core node with P1=16, J=16, L(0)=L(1)=8, L(2)=L(3)=7, L(4)=L (5)=L(6)=L(7)=6, L(8)=L(9)=L(10)=L(11)=4, and L(12)=L(14)=L (15) 2. The required number N1 of switching planes to accommodate the input links is determined as

${N1} = {{\frac{1}{16}{\sum\limits_{j = 0}^{15}{L(j)}}} = {4.875.}}$

The required number N2 of switching planes required to accommodate the output links is determined likewise, and the number N of switching planes is selected to be an integer at least equal to the higher of N1 and N2. The example of FIG. 30 is based N=5. The matrix 3000 of FIG. 30 is largely the result of the initial assignment process of collective step 2920 in FIG. 29 if the number, N, of switching planes is selected to be equal to the largest number of channels per link, leading to the configuration of FIG. 10. At step 2972 in FIG. 29 it would have been found that the input link 0 has all its channels assigned, thus terminating the subsequent assignment process. Link indices 3020 are shown as elements of the matrix 3000.

FIG. 31 illustrates the input-side connectivity matrix 3100 for the exemplary parallel-plane core node used in the example of FIG. 30. The main difference is that the number, N, of switching planes is determined as the larger of N1 and N2 as described earlier with reference to FIG. 29, step 2912. In contrast to the exemplary eight parallel-plane core node related to the matrix 3000 of FIG. 30, the exemplary five parallel-plane core node related to the matrix of 3100 of FIG. 31 has three fewer switching planes. As such, at the end of the initial assignment process of collective step 2920 in FIG. 29, there are still 14 more wavelength channels in eight input links to be assigned. The values of array α(m), 0≦m<5, after the initial assignment process would be α(0)=α(1)=16, α(2)=α(3)=12, and α(4)=8, and the number of unassigned channels in the 16 input links are L(0)=L(1)=3, L(2)=L(3)=2, L(4)=L(5)=L(6)=L(7)=1, and L(8)=L(9)=L(10)=L(11)=L(12)=L(13)=L(14)=L(15)=0.

As in the matrix 3000 of FIG. 30, link indices 3120 are shown as elements of the matrix 3100 (i.e., matrix G) of FIG. 31. For emphasis, particular link indices, an exemplary one of which is indicated as link index 3122, whose assignment was a result of the subsequent assignment process, are shown in bold face type.

FIG. 32 illustrates an exemplary two-dimensional uniform composite-star network 3200 that includes 28 edge nodes 3220. The exemplary two-dimensional uniform composite-star network 3200 is logically arranged into a plurality of “horizontal” composite-star networks 3210, each having seven edge nodes 3220 interconnected by a respective horizontal optical core 3260H. As there are four horizontal composite-star networks 3210, there are four horizontal optical cores 3260H, only one of which is illustrated. Each of the horizontal composite-star networks 3210 is of the type of composite-star network as described earlier with reference to FIG. 2.

Sets of four edge nodes 3220, each belonging to a different horizontal composite-star network 3210, are formed and the edge nodes 3220 of each set are interconnected through a respective optical core 3260L to form seven “lateral” composite-star networks 3215. As there are seven lateral composite-star networks 3215, there are seven lateral optical cores 3260L, only one of which is illustrated.

Thus, each edge node 3220 is a member of two composite-star networks 3210, 3215. The selection of edge nodes 3220 to be associated with a composite-star network 3210, 3215 can be based on expected mutual traffic among the edge nodes. The exemplary two-dimensional uniform composite-star network 3200 is described as a two-dimensional network because each edge node 3220 is a member of two composite-star networks 3210, 3215. This requires that each edge node 3220 have a sufficient number of links. Each edge node 3220 has outer links, which connect to traffic sources and traffic sinks, and inner links, which connect to optical core nodes included in the horizontal optical core 3260H and the lateral optical core 3260L.

The logical structure of the exemplary two-dimensional uniform composite-star network 3200 greatly simplifies the routing function and other control functions. In addition, this structure ensures that the vast majority of connections from a source edge node 3220 to a sink edge node 3220 will traverse at most one cross-point (intermediate) edge node 3220. This, however, requires that all edge nodes 3220 be of the same, or comparable, capacities and that each composite-star network 3210, 3215 be uniform, hence complete. In order to permit the use of edge nodes of widely-varying capacities, and core nodes of widely-varying capacities, incomplete composite-star networks, such as the network 900 of FIG. 9, may be deployed.

FIG. 33 illustrates an exemplary two-dimensional irregular (non-uniform) composite star network 3300. The exemplary two-dimensional irregular composite star network 3300 includes high capacity edge nodes 3320 and low capacity edge nodes 3325. The edge nodes 3320, 3325 are logically arranged in four non-uniform horizontal composite-star networks 3310, similar to network 900 of FIG. 9, each having several edge nodes 3320, 3325 interconnected by an optical core 3360H, only one of which is illustrated. Three sets of high capacity edge nodes 3320 are formed and the high capacity edge nodes 3320 of each set are then interconnected through a lateral optical core 3360L, thus forming three lateral composite-star networks 3315. Each of the illustrated edge nodes has one of two distinctly-different capacities, with an edge node referenced as 3320 having a relatively high capacity, hence a sufficient number of links to enable interconnection with two optical cores 3360H, 3360L. If the edge nodes 3320 of a set are of equal, or comparable, capacity, then the formed lateral composite-star networks 3315 can be uniform, hence complete, networks. The illustrated lateral composite star networks 3315 are uniform.

In the exemplary two-dimensional irregular composite star network 3300, each high capacity edge node 3320 is a member of two composite-star networks 3310, 3315. As in the network of FIG. 32, the selection of edge nodes to be associated with a composite-star network 3310, 3315 can be based on expected mutual traffic among the edge nodes.

Each low capacity edge node 3325 belongs only to one of the horizontal non-uniform composite-star networks 3310 and has links to and from one of the horizontal optical cores 3360H. A high capacity edge node 3320 belongs to both one of the horizontal non-uniform composite-star networks 3310 and one of the lateral uniform composite-star networks 3315 and has links to and from one of the horizontal optical cores 3360H and one of the lateral optical cores 3360L. Each composite-star network 3310, 3315 may be viewed as an autonomous network. The lattice structure interconnecting the plurality of horizontal non-uniform composite-star networks 3210 enables a higher-scalability than the use of a traditional full mesh to interconnect autonomous networks.

Consider a non-uniform composite-star network similar to the network 900 of FIG. 9 but having 120 symmetrical edge nodes where 16 edge nodes have nine dual links each, a dual link comprising an uplink and a downlink, with each uplink and each downlink having 32 wavelength channels, each channel carrying an optical signal modulated at 10 GB/s. Each of the remaining 104 edge nodes has seven dual links, with each uplink and each downlink having eight channels of 10 Gb/s capacity each.

The 16 edge nodes serve as high capacity edge nodes. Allocating four dual links per high-capacity edge node (i.e., 128 source ports and 128 sink ports) to traffic sources and sinks and five dual links to connect to the non-uniform optical core 3360H allows an internal expansion (5:4) that virtually eliminates internal blocking. Internal blocking, by definition, is the unavailability of a path between a free source port in any edge node and a free sink port in any other edge node. With respect to the 104 lower capacity edge nodes, allocating three dual links per lower-capacity edge node (i.e., 24 source ports and 24 sink ports) to traffic sources and sinks and four dual links to connect to the non-uniform optical core allows an internal expansion (4:3).

The-core nodes in the non-uniform core can have different reach indices. A core node in which at least one input link originates from each edge node and at least one output link terminates in each edge node is a full-reach core node, as described earlier. To avoid, or significantly reduce, tandem switching through a cross-point edge node within a non-uniform composite-star network 3310, at least one of the core nodes in the network 3310 under consideration must be a full-reach core node. Consequently, a full-reach core node must have at least one switching plane of 120×120 connectivity. As described earlier with reference to FIG. 29, the required number of switching planes is determined as: (16×32+104×8)/120=11.2, which must be rounded up to 12. Thus, 12 switch planes, of which eight switching planes are full-coverage planes and four switching planes are partial coverage planes, may be used. The assignment of channels from input links to the input ports of the switching planes, and the assignment of output ports of the switching planes to channels of output links can be determined according to the algorithm described hereinbefore with reference to FIG. 29.

The number of core nodes may be limited to four, and the fourth core node would connect to two dual links from each of the 16 high capacity edge nodes. The number of switching planes would then be: (2×16×32+104×8)/120=15.47, which must be rounded up to 16 switching planes, each having a connectivity of 120×120.

Alternatively, five core nodes may be used, of which three are full-reach core nodes, each having 12 switching planes as described above. The 104 low-capacity edge nodes are divided into two groups of 52 edge nodes each. Each of the remaining two core nodes is a partial-reach core node having a 32-channel input link from each of the 16 high capacity edge nodes, a 32-channel output link to each of the 16 high capacity edge nodes, an eight-channel input link from each of 52 low-capacity edge nodes, and an eight-channel output link to each of 52 low-capacity edge nodes. The number of ports per partial-reach switching plane is then 68 (16 ports receiving channels from the 16 high-capacity edge nodes and 52 ports receiving channels from 52 low-capacity edge nodes). The number of switching planes per partial-reach core node is determined as: (16×32+52×8)/68=13.64, which must be rounded up to 14 switching planes. An advantage of using a larger number of core nodes is a reduced fiber-distance to and from the edge nodes. A disadvantage of using a larger number of core nodes is that it can necessitate the use of partial-reach core nodes and, hence, reduced core-node connectivity. This may force a proportion of traffic to resort to tandem switching where an intermediate edge node may be used for a connection.

Where the structure of the exemplary two-dimensional irregular composite star network 3300 is applied to the parameters described above, each high capacity edge node connects to a horizontal non-uniform core and to a lateral uniform core of a lateral composite-star network. With a high-capacity edge node having 9 uplinks and 9 downlinks, each link having 32 channels, a suitable arrangement would lead to allocating 96 source ports, 96 sink ports to connect to traffic sources and traffic sinks, four uplinks and four downlinks of 32 channels each to connect to the horizontal non-uniform core and two uplinks and two downlinks of 32 channels each to connect to the lateral uniform core of a respective lateral network. The 104 lower capacity edge nodes per non-uniform network 3310 connect only to a non-uniform optical core 3360H and, as described above, three dual links per lower-capacity edge node (i.e., 24 source ports and 24 sink ports) are allocated to traffic sources and sinks and four dual links connect to the non-uniform optical core 3360H.

The horizontal non-uniform core may now have four identical optical core nodes, each having 12 switching planes of connectivity 120×120 (input ports×output ports). Each of the four identical optical core nodes is arranged to support 16 input links of 32 channels each, 104 input links of eight channels each, 16 output links of 32 channels each and 104 output links of eight channels each. Although the four optical core nodes are identical, the optical core is still termed non-uniform because at least two of the switching planes of each of the optical core nodes have a different coverage index.

A two-dimensional network having 64 non-uniform composite-star networks, with the parameters above (16 edge nodes with 96 source/sink ports each and 104 edge nodes with 24 source/sink ports each), would have a total number of source/sink ports of 258,048, and with each source or sink port operating at 10 Gb/s, the total access capacity is about 2.58 petabits per second.

The selection of geographic locations for the edge nodes, and the grouping of high capacity edge nodes to form lateral networks are inter-related. This is a combinatorial-optimization problem that may be solved using heuristics.

Routing

It is common practice to facilitate routing of data from a source edge node to a sink edge node through a given network by providing, at the source edge node, a routing table. Such a routing table preferably includes a route set associated with each sink edge node reachable by the source edge node through the given network. The route set is a listing of selected routes from the source edge node to the sink edge node. A source edge node and a sink edge node define a “node-pair” and a route set is associated with each node pair.

In a single-dimensional, non-uniform, composite-star network, the number of direct routes in a route set associated with a particular source edge node and a particular sink edge node is equal to the number of optical core nodes in the optical core to which the source edge node and the sink node connect. Additional, indirect routes, through tandem edge nodes, may also be included in the route set. Where the number of optical core nodes is relatively small, the number of routes per route set may be manageable. However, in a two-dimensional, non-uniform, composite-star network, the routing table at a source edge node may contain routes to sink edge nodes that are not connected to the source edge node directly through an optical core. The number of routes for a node-pair can be very large and, therefore, it may not be beneficial to list all the possible routes, between the edge nodes forming the node-pair, in the routing table. The derivation of route sets in a two-dimensional non-uniform, composite-star network is described below with reference to FIGS. 34 to 40.

For a given node-pair, a subset of selected routes may be listed in the associated route set in the routing table at the source edge node of the node-pair. The subset can be calculated according to certain route metrics. Route metrics can be based on factors such as administration constraints, link capacity, distance, cost and reliability. In a network having a relatively small number of edge nodes, route sets can be calculated by a network controller and distributed to controllers of the edge nodes. In a wide-coverage network having a large number of edge nodes, route set derivation may be a concerted effort of edge node controllers. Route sets need not be calculated frequently, for instance, only when the topology of the network is modified, i.e., when links and nodes are added or removed. Routes in each route set may be ordered according to a composite metric based on the aforementioned factors. A routing procedure at a particular source edge node can be used to select routes based on the ordered list of routes.

Route Set Formation

In review, in order to facilitate the process of finding a route having a sufficient free capacity to accommodate a connection request, a route set for each edge-node pair is created and sorted according to a prescribed criterion. The derivation of the route set is an “off-line” process that need only be executed whenever relevant topological changes take place. The routes in a route set are sorted according to some route merit index.

In the non-uniform composite-star network 900 of FIG. 9, a direct route is defined by a core node and the preferred route merit index is a function of the reach of the core node and the propagation delay along the route from the source edge node to the sink edge node for which the route set is created. A simple function to determine route merit index would be the product of reach and propagation delay. Thus, a first route having a reach of eight and a propagation delay of ten milliseconds and a second route having a reach of 32 and a propagation delay of 2.5 milliseconds, would have the same route merit index of 80. When two or more routes have equal route merit indices, each may be selected at random or according to a cyclic discipline.

FIG. 34 illustrates the connectivity of two edge nodes in the network 900 of FIG. 9. In particular, the connectivity is shown for a source edge node 920-5 and a sink edge node 920-12. The source edge node 920-5 can reach four core nodes 950-2, 950-3, 950-4, 950-5 while the sink edge node 920-12 can be reached only from two core nodes, namely, the fourth core node 950-4 and the fifth core node 950-5. Consequently, the route set contains only two routes, one through each of the core nodes 950-4, 950-5 that connect to both the source edge node 920-5 and the sink edge node 920-12. The fourth core node 950-4 can reach 12 edge nodes, as indicated in FIG. 34, while the fifth core node 950-5 can reach 20 edge nodes. Thus the reach indices of the two core nodes 950-4, 950-5 are 12 and 20, respectively. Where a route through the fourth core node 950-4 has a propagation delay of five milliseconds and a route through the fifth core node 950-5 has a propagation delay of 3.5 milliseconds, the route merit indices of the routes are 60 and 70, respectively (12 times 5 and 20 times 3.5). The route through the fourth core node 950-4 is therefore preferable, as the route has a lower route merit index, and would be selected if the route can accommodate the capacity specified in a connection request. In the network 900 of FIG. 34, a route set is defined by a subset of core nodes 950. A route set from a first edge node to a second edge node is not necessarily defined by the same core nodes defining the route set from the second edge node to the first edge node.

In the exemplary two-dimensional irregular composite star network 3300 of FIG. 33, the routes from an edge node 3320, 3325 within a non-uniform composite-star network 3310 to another edge node 3320, 3325 within the same non-uniform composite-star network 3310, are preferably confined to that non-uniform composite-star network. A route from a source high capacity edge node 3320 in a first non-uniform composite-star network 3310 to a sink high capacity edge node 3320 in a second non-uniform composite-star network 3310 is either direct, through the uniform core 3360L, or has two hops, either one hop through the first non-uniform composite-star network 3310 and another hop through a lateral composite-star network 3315 or one hop through the lateral composite-star network 3315 to which the source high capacity edge node 3320 belongs and another hop through the second non-uniform composite-star network 3310.

A route from a low capacity edge node 3325 in a first non-uniform composite-star network 3310 to a low capacity edge node 3325 in a second non-uniform composite-star network 3310 includes three hops. Two alternative three-hop routes are indicated in FIG. 35 for a connection between a source low capacity edge node 3325X and a sink low capacity edge node 3325Y. The first of the alternative routes begins with a hop through the first non-uniform composite-star network 3310 to a first cross-point high capacity edge node 3320P, continues with a hop through a lateral composite-star network 3315 to a second cross-point high capacity edge node 3320Q and finishes with a hop through the second non-uniform composite-star network 3310 to the sink low capacity edge node 3325Y. The second of the alternative routes begins with a hop through the first non-uniform composite-star network 3310 to a third cross-point high capacity edge node 3320A, continues with a hop through a lateral composite-star network 3315 to a fourth cross-point high capacity edge node 3320B and finishes with a hop through the second non-uniform composite-star network 3316 to the sink low capacity edge node 3325Y.

A core node in a given composite-star network has a controller which is aware of the originating edge node of each input link of the core node and the terminating edge node of each output link of the core node. The core node controller can, therefore, determine the edge-node pairs that can exchange signals through the core node. The core node controller may then inform a controller of each edge node of the other edge nodes that can be reached through the core node.

A composite-star network has a plurality of core nodes, with each core node controller providing to a controller of each edge node information about the other edge nodes that can be reached through said each core node. Each edge node controller can then construct a route set from the edge node to each other edge node in the composite-star network.

Each edge node can also estimate the one-way propagation delay along each route in each of its route sets. If a route from an edge node X to an edge node Y and a route from edge node Y to edge node X follow the same physical path, the propagation delays in the opposite directions would have a negligible difference. The one-way propagation delay is then conveniently determined by measuring a round-trip propagation delay and dividing the measurement by two. If the physical paths in the opposite directions have significantly different lengths, then other means for determining the one-way propagation delay would be needed. Techniques for measurement of propagation delay are known in the art and are not discussed herein.

In summary, within each composite-star network, a controller of each edge node can determine a route set to each other edge node, with each route set including a plurality of routes, each route being defined by a core node. The controller of each edge node also has an estimate of the propagation delay along each route in each route set. A descriptor of each route in a route set from a first edge node to a second edge node within the same composite-star network would include five fields: an identifier of a first edge node, an identifier of a core node, an identifier of a second edge node, a core-node reach index and an estimate of the propagation delay along the route from the first edge node to the second edge node through the core node.

Route Set in a Two-Dimensional Network

The exemplary two-dimensional irregular composite star network 3300 of FIG. 33 has n>1 independent non-uniform composite-star networks 3310 and m≧1 lateral composite-star networks 3315. Each non-uniform composite-star network 3310 includes both high capacity edge nodes 3320 and low capacity edge nodes 3325 and each lateral composite-star network 3315 includes only high capacity edge nodes 3320, each of which also belongs to an independent non-uniform composite-star network 3310. A route set from one edge node 3320, 3325 to each other edge node 3320, 3325 within a non-uniform composite-star network 3310 is hereinafter called a principal route set and a route set from a high capacity edge node 3320 to each other high capacity edge node 3320 within a lateral composite-star network 3315 is hereinafter called a lateral route set. A controller of a low capacity edge node 3325 stores principal route sets of the low capacity edge node 3325 within the non-uniform composite-star network 3310 to which it belongs. In addition to principal route sets, the controller of a high capacity edge node 3320, which is the intersection point of two composite-star networks 3310, 3315 stores lateral route sets for the high capacity edge node 3320 within the lateral composite-star network 3315 to which it belongs.

In order to enable each edge node to compute a route-description list (see list 4000 of FIG. 40), each high capacity edge node 3320 at the intersection of a non-uniform composite-star network 3310 and a lateral composite-star network 3315 communicates a description of all the principal route sets within the non-uniform composite-star network 3310 to each other high capacity edge node 3320 in the lateral composite-star network 3315. Thus each high capacity edge node 3320 in a lateral composite-star network 3315 has knowledge of each principal route set from each other high capacity edge node 3320 in the lateral composite-star network 3315.

Routing in a Non-Uniform Two-Dimensional Network

The methods, of route selection in the exemplary two-dimensional irregular composite star network 3300 of FIG. 33, in accordance with the present invention, include: coarse route selection, fine route selection and adaptive coarse route selection.

Coarse Route Selection

A trail is defined herein by a source edge node, a sink edge node and traversed cross-point edge nodes, if any. A coarse route set for any edge-node pair comprises at least one trail. A trail comprises one or more segments and each segment comprises parallel routes between two edge nodes through optical core nodes. A track from a source edge node to a sink edge node is a concatenation of routes, one from each segment. The trails in a coarse route set can be sorted according to a merit criterion that combines propagation delay along the shortest route between successive edge nodes and the reach index of each traversed edge node, where the reach index of a given edge node is the number of core nodes to which the given edge node connects. The sorted routes are examined sequentially with each connection request. The process of route selection based on trails and the tracks within a trail will be further illustrated with reference to FIG. 36.

Fine Route Selection

In this method, the route set includes all the tracks corresponding to each trail. The tracks are sorted according to a merit criterion that combines the propagation delay along each track and the reach index of cross-point edge nodes traversed. The routes are examined sequentially. This method requires storage of a relatively-large route-description data and can be time-consuming, resulting in a reduced connection-request processing rate. It is noted that routes in consecutive entries in the table of sorted routes may belong to different trails.

Adaptive Coarse Route Selection

This method is adapted from a selective routing method described in U.S. Pat. No. 5,629,930, issued to Beshai et al. and titled “Call routing in an ATM switching network”. In the adaptive coarse route selection method, the trails in a coarse route set are presorted according to a merit criterion. To select a route, the trails are examined sequentially, until a track having a sufficient unassigned capacity is found, for connection requests specifying a capacity requirement not exceeding a prescribed threshold, 10 megabits per second, for example. For capacity requirements exceeding the threshold, two trails, selected according to a merit criterion, are examined simultaneously. A routing message is sent along each of the selected two trails, and each edge node indicates, in a respective field in the message, the propagation delay along the selected paths. The routing message is returned to the source node by the destination node. If only one of the trails can accommodate the connection, that one trail is selected. If the two trails can accommodate the connection request, the source edge node may then select the trail offering a track of lower propagation delay. The selection may be made on the basis of more elaborate criteria, such as a function of propagation delay and reach-indices of traversed core nodes. If neither of the trails can accommodate the connection, a subsequent trail is attempted.

An edge node 3320, 3325 in the non-uniform composite-star network 3310 views each other non-uniform composite-star network 3310 as a foreign network.

Selection of Route Set Generation Method

As described earlier, there are two approaches to defining a route set. The first is based on specifying trails, thus a route set reduces to a “trail set” and the second is based on specifying tracks, thus a route set becomes a “track set”. The decision to adopt a routing discipline based on trail sets or a routing discipline based on track sets is heavily dependent on the number of nodes in the network. The routing discipline based on track sets requires storing a detailed data record for each track, as illustrated in FIG. 40, and may not be suitable for a network having a very large number of nodes, exceeding 10,000 for example, due to both the storage requirement and the signaling overhead. A threshold number of nodes, below which the use of the routing discipline based on track sets is preferred, is a design choice. Further details on the derivation of trail sets and track sets are given below.

The Trail Set Method

A first edge node has a single trail to a second edge node if the first edge node and the second edge node belong to the same non-uniform composite-star network 3310 (see FIG. 33) or the same lateral composite-star network 3315. This single trail includes only one segment. As defined earlier, a segment comprises parallel paths between two edge nodes through optical core nodes.

If the second edge node belongs to a foreign non-uniform network, a trail from the first edge node to the second edge node may be fully defined by a single high capacity edge node. The trails in a trail set are determined as follows:

-   -   (a) In a local non-uniform network 3310 having m≧1 high capacity         edge nodes, a low capacity edge node 3325 has a trail set         describing m trails, each trail having three segments. The low         capacity edge node 3325 need only include, in its trail set to         each low capacity edge node belonging to a foreign network, m         identifiers, one identifier corresponding to each of the m high         capacity edge nodes in the local non-uniform network 3310.         Although the trails from edge nodes in the local non-uniform         network to each of the edge nodes in a foreign non-uniform         network have the same m identifiers, each of the trails can have         a different trail merit index, which is included in the         description of respective trail sets. The trail merit indices         can be different because each trail has three segments and any         two trails intersect in at most two segments.     -   (b) The above low capacity edge node has one trail, which         comprises two segments, to each high capacity edge node in the         foreign non-uniform network. The low capacity edge node need         only include, in its trail set to each high capacity edge node         in the foreign non-uniform network, a single identifier. The         identifier identifies a high capacity edge node that is in the         local non-uniform network and in a lateral network through which         the high capacity edge node that is in the local non-uniform         network connects to the high capacity edge node that is in the         foreign non-uniform network. As defined earlier, the high         capacity edge node in the local non-uniform network is called         the “cross-point edge node” of the local non-uniform network and         the lateral network.     -   (c) A high capacity edge node belonging to a local non-uniform         network and a lateral network has one trail, which comprises two         segments, to each edge node in a foreign non-uniform network         that does not belong to the lateral network.

The Track Set Method

Consider a two-dimensional non-uniform composite star network having H>1 non-uniform networks, each non-uniform network having m≧1 high capacity edge nodes, resulting in m lateral networks. The j^(th) non-uniform network, 1≦j≦H, has χ(j) low capacity edge nodes 3325, m high capacity edge nodes and a(j) optical core nodes. The k^(th) lateral network, 1≦k≦m, has b(k) optical core nodes. A track may be identified by a data record. A track set is determined as follows:

-   -   (a) Only one trail is allocated from a given low capacity edge         node in the h^(th) non-uniform network to another (high capacity         or low capacity) edge node in the h^(th) non-uniform network.         The trail includes a(h) tracks, each defined by one of the a(h)         optical core nodes in the h^(th) non-uniform network. There are         (χ(h)−1+m) other edge nodes in the h^(th) non-uniform network.         The corresponding number of data records to be stored at the         given low capacity edge node is then:         a(h)×(χ(h)−1+m)  (1)     -    (b) There are m high capacity edge nodes in each of the H         non-uniform networks. A given low capacity edge node in the         h^(th) non-uniform network can reach a given high capacity edge         node in a j^(th) foreign (j≠h) non-uniform network through m         trails, with the k^(th) trail, 1≦k≦m, having a number of         permuted tracks equal to: a(h)×b(k). It is noted, however, that         although there may be numerous permuted tracks available, the         number of tracks that can be used simultaneously is the lesser         of a(h) and b(k). The corresponding number of data records to be         stored at the given low capacity edge node to cover all high         capacity edge nodes in foreign non-uniform networks is then:

$\begin{matrix} {\left( {H - 1} \right) \times {a(h)} \times {\sum\limits_{k = 1}^{m}{b(k)}}} & (2) \end{matrix}$

-   -    (c) There are m trails from a given low capacity edge node in         the h^(th) non-uniform network to each low capacity edge node in         the j^(th) foreign non-uniform network (j≠h). Each of the m         trails is defined by one of the m lateral networks. The k^(th)         trail through the k^(th) lateral network, 1≦k≦m, comprises three         segments the first segment having a(h) paths, the second segment         having b(k) paths and the third segment having a(j) paths. Thus,         the number of permuted tracks in the k^(th) trail is:         a(h)×b(k)×a(j). The total number of data records to be stored at         the given low capacity edge node to cover all low capacity edge         nodes in the (H−1) foreign non-uniform networks is then:

$\begin{matrix} {{a(h)} \times {\sum\limits_{k = 1}^{m}{{b(k)} \times {\sum\limits_{{j = 1},{j \neq h}}^{H}{{a(j)}{{\chi(j)}.}}}}}} & (3) \end{matrix}$

-   -    (d) The total number of tracks from given low capacity edge         node in the h^(th) non-uniform network is then determined from         the expression:

$\begin{matrix} {{a(h)}\left\lbrack {{\chi(h)} + m - 1 + {\left\{ {\sum\limits_{k = 1}^{m}{b(k)}} \right\} \times \left\lbrack {H - 1 + {\sum\limits_{{j = 1},{j \neq h}}^{H}{{a(j)}{\chi(j)}}}} \right\rbrack}} \right.} & (4) \end{matrix}$

Consider a two-dimensional non-uniform composite star network having 16 non-uniform networks and four lateral networks, where each non-uniform network has four core nodes, four high capacity edge nodes and 28 low capacity edge nodes and each lateral network has three core nodes. The number of tracks from a given low capacity edge node to the remaining edge nodes in the two-dimensional non-uniform composite star network can be determined from expression (4) as 81,484 (with H=16, m=4, χ(j)=28, a(j)=4, 1≦j≦H and b(k)=3, 1≦k≦m).

The total number of single-hop paths can then be determined as 243,484. With a nominal record length per single-hop path of 16 bytes, the total storage requirement at the given low capacity edge node is 3.896 megabytes.

In a network with H=64, m=8, χ(j)=120, a)=4, 1≦j<H, and b(k)=3, 1≦k≦m, the number of tracks may be determined to be 2,909,596 and the number of single-hop paths to be 8,721,724. With a nominal record length of 16 bytes per single-hop path, the storage requirement at the given low capacity edge node would be about 139.5 megabytes.

FIG. 36 illustrates routes from a source edge node 3325X to a sink edge node 3325Y in the exemplary two-dimensional irregular composite star network 3300 of FIG. 33, which includes low capacity edge nodes 3325, each connecting only to a non-uniform core 3360H and high capacity edge nodes 3320, each connecting to a non-uniform core 3360H as well as a uniform core 3360L. FIG. 36 illustrates that there are two trails from the source edge node 3325X to the sink edge node 3325Y, where each trail is defined by a sequence of edge nodes including the source edge node 3325X, the sink edge node 3325Y and two cross-point edge nodes. Each trail comprises a chain of segments, where a segment is defined by two edge nodes 3320, 3325 connecting through an optical core which may comprise several core nodes. A segment may have two or more parallel paths, each path being defined by a core node. Each trail in this example comprises three segments. There are four parallel paths (3611, 3612, 3613 and 3614) in the segment between the source edge node 3325X and the first cross-point high capacity edge node 3320P, two paths (3515, 3516) in the segment between the first cross-point high capacity edge node 3320P and the second cross-point high capacity edge node 3320Q, and four paths (3617, 3618, 3619, 3620) in the segment between the second cross-point high capacity edge node 3320Q and the sink edge node 3325Y. The optical core nodes defining the paths are omitted in FIG. 36 for clarity.

In review, a route description can be coarse, indicating only the edge nodes traversed, or fine, indicating the specific paths used. In a coarse route description, only the traversed edge nodes are indicated and, when the coarse route description is used, one of the parallel paths from a first edge node to a second edge node is selected by the edge node at the origin of each segment. Thus, to use the coarse route description is [3325X, 3320P, 3320Q, 3325Y], the source edge node 3325X may select one of paths (3611, 3612, 3613 and 3614), the first cross-point high capacity edge node, 3320P may select either of paths 3615 and 3616 and the second cross-point high capacity edge node 3320Q may select one of the four paths (3617, 3618, 3619, and 3620).

A fine route description for the trail described by the coarse route description [3325X, 3320P, 3320Q, 3325Y] provides all 32 possible tracks, including [{3611, 3615, 3617}, {3611, 3615, 3618}, . . . , {3614, 3616, 3620}].

Careful review of the exemplary two-dimensional irregular composite star network 3300, as illustrated in FIG. 36, should lead to the conclusion that there exist two trails from the source edge node 3325X to the sink edge node 3325Y. The two trails may be referred to by the coarse route descriptions [3325X, 3320P, 3320Q, 3325Y] and [3325X, 3320A, 3320B, 3325Y]. Furthermore, in the fine route description of the two trails there will be a total of 40 possible tracks (4×2×4+2×2×2).

In a network having 16 high capacity edge nodes per non-uniform composite-star network, with four core nodes per non-uniform composite-star network and two core nodes per lateral uniform composite-star network, the number of trails between two low capacity edge nodes belonging to different networks 3310 is 16 and the corresponding number of tracks is 16×4×2×4=512.

Each non-uniform composite-star network may be prearranged to cover a specific geographical area or represent a single administrative domain. Preferably, the high capacity edge nodes 3320 that are connected to two optical cores will be the edge nodes of higher capacity, relative to other edge nodes in the same non-uniform composite-star network.

The partition of a network into geographical areas and administrative domains is facilitated by the use of the two-dimensional, non-uniform, composite-star network architecture. A single-dimensional, non-uniform, composite-star network can cover a smaller geographical area or be under the control of a single administrative domain. For example, a horizontal single-dimensional, non-uniform, composite-star network 3310 formed by the edge nodes 3320, 3325 connected to the optical core 3360H may cover the state of California. The lateral single-dimensional, uniform, composite-star network 3315 formed by the high capacity edge nodes 3320 connected to the lateral optical core 3360L may be a transcontinental backbone of a first network provider, interconnecting single-dimensional, non-uniform, composite-star networks 3310 from various areas. It is desirable that most traffic be exchanged through direct optical connections, with a small proportion of the traffic transferred to destination sink nodes through cross-point edge nodes.

Advantageously, the use of single optical hops provided by two-dimensional, composite-star networks allows the use of TDM in the optical domain, increasing the effective reach of the edge nodes. As well, confining a single-dimensional, non-uniform composite-star network to a small geographic area keeps fiber distances short and propagation delays low.

FIG. 37 illustrates a trail 3700 in a two-dimensional network, comprising three segments, from a source edge node 3720A to a sink edge node 3720D through a first cross-point edge node 3720B and a second cross-point edge node 3720C. The first segment is defined by the source edge node 3720A and the first cross-point edge node 3720B, and has four parallel paths defined by core nodes 3740 p, 3740 q, 3740 r and 3740 s. The second segment is defined by the first cross-point edge node 3720B and the second cross-point edge node 3720C and comprises two parallel paths defined by core nodes 3740 t and 3740 u. The third segment is defined by the second cross-point edge node 3720C and the sink edge node 3720D and has three parallel paths defined by core nodes 3740 v, 3740 w and 3740 x.

The propagation delays along each path in FIG. 37 and the reach indices of the associated core nodes are indicated in a table 3800 in FIG. 38 which illustrates a part of a routing decision table for one trail between the source edge node 3720A and the sink edge node 3720D. Each element of a first column 3810 of table 3800 of FIG. 38 identifies a segment of the trail. Elements of a second column 3820 define the parallel paths for each segment by a core node reference letter. A third column 3830 lists the reach index of each of the optical core nodes referenced in the second column 3820. A fourth column 3840 indicates the one-way propagation delay for each route defined by the core node referenced in the second column 3820. A fifth column 3850 indicates the route merit index of each route, the route merit index being defined, in this example, as the product of a reach index of a core node defining a route and the propagation delay along the route. A sixth column 3860 is the only column to hold state-dependent information. The sixth column 3860 indicates the available free capacity along each route. This entry is only computed when needed. The capacity may be defined in terms of wavelength channels, in a channel-switching network, or may be defined in terms of available time slots per TDM frame in a TDM switching network.

A table 3900 illustrated in FIG. 39 lists the 24 possible tracks between the source edge node 3720A and the sink edge node 3720D through the single trail 3700 of FIG. 37. A first column 3910 defines each track according to all the nodes, edge or core, that the track traverses. A second column 3920 illustrates the calculation of a track merit index for each track.

A track set 4000 illustrated in FIG. 40 lists the tracks from the table 3900 of FIG. 39 sorted in an ascending order according to the track merit index. The vacancy along each track, determined by the single-hop path that has the least vacancy, is also indicated. The vacancy in this example is given as a number of unassigned time slots. If a connection request arrives at the source edge node 3720A requiring 12 time slots, a review of the first rows of the track set 4000 of FIG. 40 yields the information that neither of the first two tracks, A-q-B-t-C-v-D or A-q-B-t-C-x-D, can accommodate the connection request, as each track has only eight free time slots. The third track, having a vacancy of 34 time slots, would then be selected, by the source edge node 3720A, to accommodate the connection request.

When a master track set, made up of the contents of multiple track sets such as the track set 4000 illustrated in FIG. 40, is too large to maintain, routing decisions may be made based on a trail set. As discussed hereinbefore, the trail set includes a trail merit index associated with each trail and each trail is associated with a plurality of tracks from the source edge node to the sink edge node. The trail having the optimum trail merit index may be selected as a candidate trail to satisfy a connection request. A trail merit index for a given trail is based on the track merit indices of the plurality of tracks associated with the given trail. Once a candidate trail has been selected, a candidate track may be selected having the optimum track merit index among the tracks associated with the candidate trail. The vacancy of the candidate track is then determined and, where the vacancy equals or exceeds the required capacity specified in the connection request, an acceptance indication indicating an acceptance of the connection request may be transmitted to the origin of the connection request.

The trail merit index for a given trail may be determined in a number of different ways. For instance, the trail merit index for a given trail may be determined as an optimum track merit index among the track merit indices of the plurality of tracks associated with the given trail. Alternatively, the trail merit index for a given trail may be determined as an average of track merit indices of each of the plurality of tracks associated with the given trail. Additionally, the trail merit index for a given trail may be determined as a weighted average of track merit indices of each of the plurality of tracks associated with the given trail.

Advantageously, the use of an optical core network having diverse nodal capacities allows for the use of edge nodes of diverse nodal capacities and the deployment of a partial, non-uniform, composite-star network long before a need develops for a complete, uniform, composite-star network.

The use of network routes traversing consecutive optical core nodes leads to a rigid inefficient network. Uniform complete networks using composite-star structures provide agility, high performance and high efficiency. The structure enables the use of fine granularity of capacity allocation. However, there is further room for optimizing the overall length of optical links if the network is incomplete, utilizing edge nodes of dissimilar capacity and reach and core nodes of different capacity and reach.

Other modifications will be apparent to those skilled in the art and, therefore, the invention is defined in the claims. 

1. An optical core node comprising: a plurality of optical switching planes, at least two of said switch planes have different coverage indices, a coverage index of an optical switching plane being based on a number of input links and output links with which the optical switching plane communicates; a plurality of demultiplexers, each adapted to receive a signal on an input link, demultiplex said signal into a plurality of wavelength channels and connect at least one of said plurality of wavelength channels to at least one of said plurality of optical switching planes; and a core node controller adapted to receive a connection request, where said connection request specifies a specific input link, a specific output link, and a requested capacity, said core node controller operable to: identify a subset of optical switching planes, in said plurality or optical switching planes, where each of said optical switching planes in said subset has an unassigned path from said specific input link to said specific output link; and select a candidate optical switching plane, where said candidate optical switching plane has the least coverage index in said subset of optical switching planes.
 2. The optical code node of claim 1 wherein at least one of said optical switching planes connects to a first number of input links and a second number of output links where said second number differs from said first number.
 3. The optical core node of claim 1 wherein said demultiplexers are independent of each other and at least two of said demultiplexers output a different number of wavelength channels each connecting to one of said optical switching planes.
 4. A method of selecting an optical switching plane to satisfy a connection request within an optical core node having a plurality of optical switching planes, each optical switching plane having a respective number of input ports and a respective number of output ports, each channel of each input link in a plurality of multi-channel input links connecting to an input port in one of said optical switch planes and each channel of each output link in a plurality of multi-channel output links connecting to an output port in one of said optical switch planes, where said connection request includes a specified input link from said plurality of multi-channel input links, a specified output link from said plurality of multi-channel output links, and a requested capacity, said method comprising: determining a coverage index for said each optical switching plane, where a coverage index for said each optical switching plane is a metric based on said respective number of input ports and said respective number of output ports; identifying a subset of optical switching planes, in said plurality of optical switching planes, where each optical switching plane in said subset is connected to said specified input link and said specified output link; and selecting a candidate optical switching plane, where said candidate optical switching plane has the least coverage index in said subset of optical switching planes.
 5. The method of claim 4 wherein said respective number of input ports differs from said respective number of output ports.
 6. The method of claim 4 wherein said coverage index is determined as the lesser of said respective number of input ports and said respective number or output ports.
 7. The method of claim 4 wherein said coverage index is determined as the arithmetic mean of said respective number of input ports and said respective number of output ports.
 8. The method of claim 4 wherein said coverage index is determined as the geometric mean of said respective number of input ports and said respective number of output ports.
 9. The method of claim 4 wherein at least one of said optical switching planes connects to all said multi-channel input links and all said multi-channel output links.
 10. A method of selecting a candidate optical switching plane through which to route a connection within an optical core node having a plurality of optical switching planes, a plurality of input links, and a plurality of output links, where said selecting is responsive to receiving, at said optical core node, a connection request specifying a specified source node, a specified sink node and a required number of wavelength channels, said method comprising: identifying a source input link, from said plurality of input links, originating from said specified source node; identifying a sink output link, from said plurality of output links, terminating on said specified sink node; identifying a subset of optical switching planes, in said plurality of optical switching planes, where said subset includes those optical switching planes that are connected to said source input link and said sink output link; selecting said candidate optical switching plane from said subset of optical switching planes, such that said candidate optical switching plane has a least coverage index among said subset of optical switching planes, a coverage index of a given optical switching plane being a metric based on a number of input links and a number of output links to which said optical switching plane connects; determining a number of available input ports, of said candidate optical switching plane, connected to said source input link; determining a number of available output ports, of said candidate optical switching plane, connected to said sink output link; determining a number of allocable paths through said candidate optical switching plane as the least of said number of available input ports, said number of available output ports and said required number of wavelength channels; and where said number of allocable paths is equal to said required number of wavelength channels, sending an acceptance indication to said specified source node indicating an acceptance of said connection request, where said acceptance indication includes an identification of wavelength channels corresponding to said candidate optical switching plane.
 11. The method of claim 10 wherein said number of allocable paths is less than said required number of channels, further comprising: determining a remaining number of required wavelength channels by reducing said number of required channels by said number of allocable channels; selecting a second candidate optical switching plane from said subset of optical switching planes, such that said second candidate optical switching plane has a second least coverage index among said subset of optical switching planes; determining a second number of available input ports, of said second candidate optical switching plane, connected to said source input link; determining a second number of available output ports, of said second candidate optical switching plane, connected to said sink output link; determining a second number of allocable paths through said second candidate optical switching plane as the least of said second number of available input ports, said second number of available output ports and said remaining number of required wavelength channels; where said second number of allocable paths is equal to said remaining number of required wavelength channels, sending an acceptance indication to said specified source node indicating an acceptance of said connection request, where said acceptance indication includes an identification of said wavelength channels corresponding to said candidate optical switching plane and wavelength channels corresponding to said second candidate optical switching plane.
 12. The method of claim 10 wherein said plurality of input links and output links are connected to said switching planes according to a discipline such that said subset of optical switching planes comprises optical switching planes that are consecutive in a coverage index-based ranking of said plurality of optical switching planes.
 13. An optical core node receiving optical signals on a plurality of input links and transmitting optical signals on a plurality of output links, said optical core node comprising: a plurality of optical switching planes, wherein at least two of said plurality of optical switching planes have a different coverage index, where a coverage index of a given optical switching plane is a metric based on a number of links in communication with said given optical switching plane; and a core node controller adapted to: receive a connection request, where said connection request specifies a specified source node, a specified sink node and a requested number of wavelength channels; identify a subset of optical switching planes, where said subset includes all optical switching planes, of said plurality of optical switching planes, that receive at least one wavelength channel originating at said specified source node and transmit at least one wavelength channel to terminate at said specified sink node; select a candidate optical switching plane in said subset of optical switching planes, where said candidate optical switching plane has a least coverage index among said optical switching planes in said subset; determine a number of available input ports, of said candidate optical switching plane, connected to said specified source node; determine a number of available output ports, of said candidate optical switching plane, connected to said specified sink node; determine an allocable number of paths as the least of said number of available input ports, said number of available output ports and said required number of wavelength channels; and where said allocable number of paths is equal to said number of required paths, transmit an acceptance indication to said specified source node indicating an acceptance of said connection request, where said acceptance indication includes an identity of wavelength channels corresponding to said candidate optical switching plane.
 14. The optical core node of claim 13 wherein said coverage index of each of said plurality of optical switching planes is equal to a degree of said each of said plurality of optical switching planes, where a degree of a given optical switching plane is a metric based on a number of ports associated with said given optical switching plane, including input ports and output ports.
 15. A method of generating a schedule for a connection within an optical core node having a plurality of optical switching planes, where said generating is responsive to receiving, at said optical core node, a connection request specifying a specified source node, a specified sink node and a required number of time slots in a time division multiplexed frame, said method comprising: identifying a subset of said plurality of optical switching planes, where said subset includes all optical switching planes, of said plurality of optical switching planes, that receive at least one wavelength channel from said specified source node and transmit at least one wavelength channel to said specified sink node; selecting a first candidate optical switching plane in said subset of optical switching planes, such that said first candidate optical switching plane has a minimum coverage index among said optical switching planes in said subset, where a coverage index of a given optical switching plane is a metric based on a number of links, including input links and output links, with which said given optical switching plane communicates; and for a first time slot in said time division multiplexed frame and for said first candidate optical switching plane: determining a number of available input ports in communication with said specified source node; determining a number of available output ports in communication with said specified sink node; and determining an allocable number of paths as the least of said number of available input ports, said number of available output ports and said required number of time slots.
 16. The method of claim 15 where said allocable number of paths is less than said required number of time slots, further comprising: reducing said required number of time slots by said number of allocable paths to give a remaining number of required time slots; and selecting a second candidate optical switching plane in said subset of optical switching planes, such that said second candidate optical switching plane has a second least coverage index among said optical switching planes in said subset.
 17. The method of claim 16 while said remaining number of required time slots is greater than zero, further comprising selecting candidate optical switching planes in said subset in order from least coverage index to greatest, determining allocable numbers of paths for candidate optical switching planes and, once all said optical switching planes have been selected, repeating said selecting for a time slot subsequent, in said time division multiplexed frame, to said first time slot.
 18. A method of generating a schedule for a connection within an optical core node having a plurality of optical switching planes, where said generating is responsive to receiving, at said optical core node, a connection request specifying a specified source node, a specified sink node and a required number of time slots in a time division multiplexed frame, said method comprising: identifying a subset of said plurality of optical switching planes, where said subset includes all optical switching planes, of said plurality of optical switching planes, that receive at least one wavelength channel from said specified source node and transmit at least one wavelength channel to said specified sink node; and selecting a first candidate optical switching plane in said subset of optical switching planes, such that said first candidate optical switching plane has a minimum coverage index among said optical switching planes in said subset, where a coverage index of a given optical switching plane is a metric based on a number of links, including input links and output links, with which said given optical switching plane communicates; wherein said plurality of optical switching planes are logically arranged in a sorted order so that said subset comprises q optical switching planes indexed consecutively as optical switching planes 1 through q.
 19. The method of claim 15 where said allocable number of paths is less than said required number of time slots, further comprising: reducing said required number of time slots by said number of allocable paths to give a remaining number of required time slots; and selecting a second time slot in said time division multiplexed frame; for said second time slot and for said first candidate optical switching plane: determining a second number of available input ports in communication with said specified source node; determining a second number of available output ports in communication with said specified sink node; determine a second allocable number of paths as the least of said second number of available input ports, said second number of available output ports and said remaining number of required time slots; and where said second allocable number of paths is equal to said remaining number of required time slots, sending an acceptance indication to said specified source node indicating an acceptance of said connection request, where said acceptance indication includes identities of said first and second time slots, and an identity for each wavelength channel corresponding to said available input ports at said first candidate optical switching plane.
 20. The method of claim 19 further comprising, while said remaining number of required time slots is greater than zero, selecting subsequent time slots in said time division multiplexed frame, determining allocable numbers of paths for said first candidate optical switching plane and, once all said subsequent time slots in said time division multiplexed frame have been selected, repeating said selecting for subsequent optical switching planes in said subset in order from least coverage index to greatest coverage index.
 21. An optical core node comprising: a plurality of demultiplexers, each of said demultiplexers demultiplexing one of a plurality of input links, each said input link having multiple wavelength channels, into a plurality of input wavelength channels; a plurality of optical switching planes, each connecting to at least one input wavelength channel from at least one of said input links, wherein at least two of said plurality of optical switching planes have a different coverage index, a coverage index of a given optical switching plane being a metric based on a number of said input links to which said given optical switching plane connects; a plurality of multiplexers, each of said plurality of multiplexers multiplexing a plurality of output wavelength channels from at least one of said plurality of optical switching planes onto one of a plurality of output links, each said output link having multiple wavelength channels; and a core node controller adapted to: receive a connection request, where said connection request specifies a specified input link, a specified output link and a requested capacity; identify a subset of optical switching planes, where said subset includes all optical switching planes, of said plurality of optical switching planes, that connect to said specified input link and said specified output link; select a candidate optical switching plane in said subset of optical switching planes, where said candidate optical switching plane has a least coverage index among said optical switching planes in said subset; and determine availability of a path from an input wavelength channel in said specified input link to an output wavelength channel in said specified output link through said candidate optical switching plane for a particular time slot in a time division multiplexed frame.
 22. The optical core node of claim 13 further comprising: a plurality of demultiplexers, each of said plurality of demultiplexers adapted to receive an optical signal from a source node on one of said plurality of input links, demultiplex said optical signal into a plurality of wavelength channels and transmit at least one of said plurality of wavelength channels to at least one of said plurality of optical switching planes; and a plurality of multiplexers, each of said plurality of multiplexers adapted to receive a plurality of wavelength channels from at least one of said plurality of optical switching planes, multiplex said plurality of wavelength channels into an outgoing optical signal and transmit said outgoing optical signal on one of said plurality of output links to a sink edge node.
 23. The method of claim 15 further comprising: where said allocable number of paths is equal to said required number of lime slots: sending an acceptance indication to said specific source node indicating an acceptance of said connection request, where said acceptance indication includes a schedule including an identity of said first time slot and an identity for each wavelength channel corresponding to said available input ports at said first candidate optical switching plane.
 24. The method of claim 16 further comprising: for said first time slot and for said second candidate optical switching plane: determining a second number of available input ports in communication with said specified source node; determining a second number of available output ports in communication with said specified sink node; determining a second allocable number of paths as the least of said second number of available input ports, said second number of available output ports and said remaining number of required time slots; and where said second allocable number of paths is equal to said remaining number of required time slots, sending an acceptance indication to said specified source node indicating an acceptance of said connection request, where said acceptance indication includes an identity of said first time slot and an identity for each wavelength channel corresponding to said available input ports at said first candidate optical switching plane and said available input ports at said second candidate optical switching plane. 